leaving png aside...since it's not yet 100% supported...you should really be using both gif and jpg...they do different things..gif is good for things like text, buttons and similar where you have few colors and "sharpness" is important...because of jpg's lossy compression, it is not good for things with sharp, defined edges...they tend to look blurred and hard to read...jpg on the other hand is great for images which have naturally blurred edges...like a photograph...also if you want to do fancy web tricks like mouse overs...jpg isn't good beacuse you often want transparency in the graphics with the onmouse events...



HERE'S a good intro to web graphics....