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JPEGs use 'lossy' compression, discarding color information while compressing. In general, a JPEG will compress a photographic image two to three times smaller than GIF. For flat-color graphics, however, GIF is far more efficient than JPEG. Because they need to be decompressed, JPEGs may take longer to render that GIFs, the files being of equal size. As a general rule, you can compress a JPEG from 10-40% without a noticeable loss of quality when viewing on the Web. You should compress JPEGs only once, for the color information is discarded. If you compress a JPEG and then edit it, and then compress it again, you are actually degrading the image, because the second loss of information is added to the first loss. Always work from a 'master' JPEG when editing. With JPEGs, flat colors can render pixelated or blotchy, and sharp edges blur. The JPEG format also allows insertion of meta-data into the image file; allowing you to store information such as copyright, caption, keywords etc.
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