PNG and GIF use lossless compression, JPEG uses lossy compression.
JPEG is inappropriate for pixel-based images, bitmaps, screen captures, etc. If you use JPEG on images with sharp edges the lossy compression introduces aliasing. It is appropriate for photographs. A man of words and not of deeds is like a garden full of weeds; a man of deeds and not of words is like a garden full of turds — Anonymous
But I have an awful image on the front page at the moment, I should do something about that... :\
Run, run, run!!!