Uh, I've been wondering this for a while, but how many of you actually clicked the images in the first post? I'm mostly referring to Wudge and King Boo here - seeing as he used the attachment system to showcase his sprites, what you see are thumbnails. Clicking on them will show the proper uploaded image, which btw King Boo, are saved as PNG's.
Image compression doesn't change the dimensions of the image, so your conclusion about not saving as GIF or PNG is incorrect. Furthermore, transparent backgrounds aren't everything. You only need to post your link once.