Well, you can still do it with one image (CSS sprites and kind of "sliding doors" technique), however an image is one more resource to maintain, and that "truckload" of CSS in reality is smaller than HTTP request headers of an addtional request for said image (even if responese is 304 Not Modified). Sure, you can properly set Expires into far future and save request for repeated visitors, but then you will need to come with a versioning scheme. All in all, CSS way is simpler and easier to maintain. Add some more bling with CSS transforms, e.g. some nifty color fade/shift on hover and it gets realy tough for an image based solution to stand a chance.