The example was impressively slow for me in chrome. There's quite a delay between the time you place the cursor above a button, and the time the hover image actualy display. Even once the image was downloaded, it kept on being slow.
Also, still in the example and both in Chrome and Firefox I didn't see the image change on click (like a pushed down image).
The image apparently just go back to the normal image as not being hovered. Is that the example that is coded like that ("missing" clicked image) or is it another trouble ?
FF12 and chrome 18 up to date.