GIF or JPG image formats? Which should I use?

GIF is a compression scheme that will preserve dots and sharp boundary contrasts, as on text. When an image is cartoonish with a few colors (4, 16, 64), GIF compression can tremendously shrink the file size, without losing crispness, by reducing the GIF color palette from 256 to the smallest reasonable number of colors.

The GIF format is also used to drop out the background to make it transparent. The image can then be overlaid on a patterned or colored background. The JPG format does not support a transparent layer.

JPG compression is generally used with photographic images where there are thousands of minute color changes across the image and where all those colors need to be there for a smooth transition. The compression algorithm shrinks file sizes by averaging out and blending some colors, so the image will be fuzzier and color boundaries may blur some. The smoothing effect is generally desirable in photos.

JPG compression can greatly reduce the file size of large images, depending on the amount of compression that is applied.

Examples:

Below are visual examples of the differences between GIF and JPG. Beside each image is a blowup of the details to show the pixels.

Images that should use the GIF format:

Images that should use the JPG format:

Design Considerations

There are some design considerations for saving images in the wrong format (not recommended for displaying images on the web). A photographic image that should be saved as a JPG, but that is saved as a GIF image, can usually be converted to a JPG image (without suffering too much in appearance) if no palette compression is used when saving the GIF (all 256 colors are available). A graphic that should be saved as a GIF, but that is saved in the JPG format, can be converted to a GIF (without blurry edges) if no compression is used (or 100% quality is selected) when saving the JPG file.