There seems to have been some confusion amongst you about this. So, just a very quick post on which ones to use and when:
GIF: Will only use up to 256 colors when saving, so this works best for solid colored things (preferably with aliased edges) that have few colors and no gradients. You can also have transparency with GIF, however it will hardly ever look right, will give you some very nasty jagged outlines unless you are using it on straight edged shapes such as bitmap fonts.
JPEG: The one which you will probably end up using the most, works very well with all types of images, in that it uses millions of colors when saving - except that there is no transparency.
PNG: There are two types to this - PNG8 and PNG24. Avoid 8 like the plague! Use GIF instead, works the same way, with only 256 colors. The one to go for is PNG24, which is a fabulous image format that also works with millions of colors + transparency. So, you cannot go wrong with this. Downside: Will create large file sizes when used on large, complex textured images, in which case JPEG is a much better option.
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