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Convert Images to Different Formats with Pillow

Learn how to use Python Pillow to open images and save them as JPEG, PNG, GIF, or PDF, including mode conversion, transparency, quality, and verification.

What Image Format Conversion Means

Image conversion means reading an existing source image, decoding its pixel data, and writing a new output image using a different image format, such as JPEG, PNG, GIF, or PDF.

The source filename identifies the file Pillow reads. The destination filename identifies where Pillow writes the converted file. The destination format controls how the image is encoded.

source.jpg  ->  Pillow reads and decodes the image  ->  output.png

A file extension is only the suffix of a filename, such as .jpg or .png. Renaming photo.jpg to photo.png does not convert the image. The file still contains JPEG data, and programs may fail to open it or interpret it correctly. A real conversion requires Pillow to decode the source and encode new output data.

Install Pillow

Pillow is a Python imaging library commonly imported through the PIL package namespace. Install it with:

python -m pip install Pillow

These examples assume basic Python syntax, imports, file paths, and package installation.

Open an Image with Pillow

Import Image from PIL, then call Image.open(). This method loads an image from a filename or file-like object. Pillow generally identifies the source format from the image data rather than trusting the filename extension.

from PIL import Image

image = Image.open("source.jpg")
print(image.format)
print(image.size)
print(image.mode)

The format property commonly reports a value such as JPEG or PNG. The size property contains the width and height as a tuple, and mode describes the pixel representation.

Save an Image in a New Format

Use the image object's save() method to create the output file. Its first argument is the destination path or filename. Pillow can often infer the output encoder from the destination extension.

from PIL import Image

image = Image.open("source.jpg")
image.save("output.png")

Because the destination ends in .png, Pillow selects PNG output. The source extension and destination extension can be different.

You can also provide an explicit format. This is useful when the destination has no recognized extension, has an ambiguous extension, or when you want the encoder choice to be clear.

from PIL import Image

image = Image.open("source.jpg")
image.save("output.gif", format="GIF")

In this example, format="GIF" requests the GIF encoder even though the destination filename already suggests it.

Common Pillow Output Formats

Practical Conversion Examples

Convert JPEG to GIF

This example demonstrates the basic open-and-save workflow with an explicit target format.

from PIL import Image

image = Image.open("photo.jpg")
image.save("photo.gif", format="GIF")

GIF is most suitable when the result can tolerate an indexed palette. A photograph may lose smooth color gradients because GIF supports only a limited number of colors.

Convert PNG to JPEG

JPEG cannot store an alpha channel. If the PNG is an RGBA image, directly saving it as JPEG may raise an error or require a deliberate mode conversion. The simplest preparation is to discard transparency by converting to RGB:

from PIL import Image

image = Image.open("source.png").convert("RGB")
image.save("output.jpg", format="JPEG", quality=90)

This creates a JPEG with no transparency. Transparent pixels cannot remain transparent in the JPEG file, so their appearance must be handled before saving.

Flatten Transparency onto a Background

If transparent pixels should appear against a particular color, composite the image onto a background before converting to RGB. White is a common choice, but select a color appropriate for the final design.

from PIL import Image

source = Image.open("transparent.png").convert("RGBA")
background = Image.new("RGB", source.size, "white")
background.paste(source, mask=source.getchannel("A"))
background.save("flattened.jpg", format="JPEG", quality=90)

Use PNG instead when transparency must remain intact:

from PIL import Image

image = Image.open("transparent.png")
image.save("preserved.png", format="PNG")

Save a Photograph as PNG

PNG uses lossless compression, so it does not discard pixel information in standard PNG encoding. However, a PNG version of a photograph is often larger than the JPEG version.

from PIL import Image

image = Image.open("photo.jpg")
image.save("photo.png")

The dimensions normally remain the same, but the file size and encoding characteristics can change substantially.

Use Destination Extension Inference

When the destination has a recognized suffix, the extension can select the encoder:

from PIL import Image

image = Image.open("source.jpg")
image.save("output.png")

If the destination has no recognized extension, provide the format explicitly:

from PIL import Image

image = Image.open("source.jpg")
image.save("converted-image", format="PNG")

Save an Image as PDF

Pillow can save images as PDF when PDF support is available in the installed environment. RGB is a useful target mode for this workflow.

from PIL import Image

image = Image.open("source.png").convert("RGB")
image.save("output.pdf", format="PDF")

PDF is an image-container output option rather than a direct replacement for every image format. Check the resulting file in the application that will consume it, especially when document layout, multiple pages, or print quality matters.

Image Modes and Compatibility

An image mode describes how Pillow represents each pixel:

• RGB stores red, green, and blue color channels.
• RGBA stores red, green, blue, and an alpha channel for per-pixel transparency.
• L stores grayscale or luminance values.
• P stores palette indexes that refer to a color table.
• CMYK stores cyan, magenta, yellow, and black channels, commonly used in print workflows.

Destination formats impose restrictions. JPEG commonly expects RGB or grayscale data and cannot store an alpha channel. GIF uses an indexed palette. Some PDF workflows work most predictably with RGB or grayscale images.

Use convert() when a destination requires a particular mode:

rgb_image = image.convert("RGB")
gray_image = image.convert("L")
rgba_image = image.convert("RGBA")

Transparency and Format Limitations

The alpha channel stores transparency information. In an RGBA image, the A component controls how visible each pixel is. PNG can preserve this information, while JPEG cannot.

When JPEG output is required, choose one of two strategies:

1. Discard transparency with convert("RGB"), accepting the resulting background behavior.
2. Flatten the image against a selected background color before converting to RGB.

Do not assume that every destination preserves all source features. Conversion may alter transparency, metadata, color representation, palette details, or animation frames.

Quality and Output Options

save() accepts format-specific encoder options. The available options depend on the selected format and Pillow version.

• JPEG: The quality option controls the balance between visual detail and file size. Higher values generally retain more detail but create larger files. The optimize option may help Pillow produce a more efficient JPEG encoding, with additional processing cost.
• PNG: PNG is lossless, so compression does not intentionally discard pixel data. Compression settings can still affect output size and encoding time. A larger compression setting may require more processing.
• GIF: GIF is restricted to an indexed palette of up to 256 colors per frame. It can be useful for simple graphics and animation, but is usually a poor target for photographs.

from PIL import Image

image = Image.open("photo.png").convert("RGB")
image.save("photo.jpg", format="JPEG", quality=85, optimize=True)

Quality settings are not universal across formats. Compare the output visually and measure file sizes using representative images rather than assuming one setting is best for every file.

Verify the Conversion

A successful call to save() does not by itself confirm that the output meets your requirements. Check that the file exists, reopen it, and inspect its format, dimensions, and mode.

from pathlib import Path
from PIL import Image

output_path = Path("output.png")

if not output_path.exists():
    raise FileNotFoundError(output_path)

with Image.open(output_path) as result:
    print("format:", result.format)
    print("size:", result.size)
    print("mode:", result.mode)

Also inspect the image visually. Compare transparency, colors, sharpness, animation behavior, and metadata when those features matter. Dimensions often remain unchanged during a straightforward format conversion, but the target format can change how colors, palettes, alpha data, metadata, or multiple frames are represented.

Troubleshooting Conversion Problems

RGBA image cannot be saved as JPEG

Cause: JPEG does not support an alpha channel.

Resolution: Save as PNG to retain transparency, or composite against a background and convert to RGB before saving as JPEG.

Pillow cannot determine the output format

Cause: The destination has no recognized extension and no explicit format was provided.

Resolution: Use a recognized destination extension or pass format="PNG", format="JPEG", or another appropriate format to save().

The converted GIF looks degraded

Cause: GIF uses a limited indexed color palette.

Resolution: Use PNG or JPEG for images containing many colors, gradients, or photographic detail.

The output file is unexpectedly large or small

Cause: Formats use different compression methods, and image content strongly affects compression results.

Resolution: Select a format suited to the image and adjust supported options such as JPEG quality. Always inspect both visual quality and file size.

The image looks different after conversion

Cause: The target may not support the source's color mode, transparency, metadata, palette, or animation features.

Resolution: Inspect the source mode, convert deliberately, reopen the output, and test it in the viewer or downstream system that will use it.

Conversion Checklist

1. Identify the source image and its actual format.
2. Choose a destination format based on transparency, color complexity, animation, quality, and file-size needs.
3. Open the source with Image.open().
4. Convert the image mode when the destination format requires it.
5. Call save() with a destination filename or an explicit format.
6. Set format-specific options such as JPEG quality when appropriate.
7. Confirm that the output file exists.
8. Reopen the output and inspect its format, dimensions, mode, and visual appearance.

For related Pillow work, see the image format conversion guide as a reference while applying these patterns to your own files.