For .gif, I always thought it was a question of whether the owner of the format (is it a subsidiary of Unisys?) would eventually make infringement trouble for people using .gif images on the web. Beyond that, I believe that .png uses lossless compression to get smaller file sizes, as opposed to .jpg where the compression algorithm causes some quality loss. Both .png and .jpg make smaller file sizes than .gif, but only .gif has an animated version.
What we really need is universal support for .svg images! Right now it's very hit-or-miss because it's not in widespread enough use to be worth supporting. Pity. It's text-based and makes very efficient file sizes and is vector rather than raster so you get perfect curves and no anti-aliasing problems.
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