Regarding his whole point of using css, that upvote image should really be a background in the css file and the innerHTML should be text with a negative indent.
Or we could go one step further and replace images with unicode characters: ▲ + ▼
▲ - U+25B2 BLACK UP-POINTING TRIANGLE
▼ - U+25BC BLACK DOWN-POINTING TRIANGLE
Just please either don't put them right next to each other or make the vote reversible. I've often accidentally hit the wrong arrow.