Compare (taken from http://en.wikipedia.org/wiki/Wikitex):
http://users.aber.ac.uk/jqh1/chess.gif (17.1k)
With the png version I just made:
http://users.aber.ac.uk/jqh1/chess.png (7.9k)
That's 46% of the original size by switching format. Futhermore, doesn't wikipedia prefer PNG?
Jim