.PNG isn't transparent in Internet Explorer. .GIF for IE. Like someone said already, .jpeg at 12 quality is pretty much the same quality as a .PNG but .PNG takes longer to load on a slower connection.
I save it as png, but my friend can't see it and have to download it, so I save it as jpeg sometimes.
for wht i last read sonewhere heres the best use of the extensions PNG - for transparency (dosnt work in IE) jpg- for standard still image gif - for animation of course
