I believe there are ways to change images when the browser displays them, a la find and replace. (Although unfortunately that might somehow mangle the mechanics of how upvoting/downvoting/etc works.)
I’m sure we could find a suitable plugin/script/etc, or get somebody to make one.
MODS! You hear the cries of a many, and the positive impact that they have made, tis be the time to make the ponified icons by Arif a permanent item on the site.