[Hellstryker]

Since of people have been confusing compressed png with compressed jpeg, etc etc I figured I would clear this up right now for all the noobies out there.

JPEG - Most commonly used image format, due to it's compression, but creates lots of ugly looking http://en.wikipedia.org/wiki/JPEG_artifacts
PNG - Sadly probably one of the least used formats. It can't compress quite as good as JPEG, but it's completely lossless
BMP - Don't use this. Period. It is big, bulky, and annoying.
GIF - Again, don't use this, unless you need something animated. It will look quite ugly, and you don't have nearly as many features as PNG

Sticky, anyone?

[CP5670]

GIF is better than JPEG for certain types of pictures that contain a lot of text. The drop to 256 colors is preferable to the sludgy look of JPEG on such images, especially if the color reduction is done using a good dithering algorithm like octree.

[Colonol Dekker]

I prefer .png whenever possible. But i post .JPG's out of habit.