This is my test page:
The issue I am seeing appears to only happen in Firefox. When you mouseover the square Quiz graphic, that image fades out. As this happens, the image appears to jump one pixel to the left. When you mouseout, the same thing seems to happen as the image fades back in. After the animation is complete, the image returns to the correct position (where it started out).
I think this problem is related to the width of the browser window. Sometimes, you don't see the problem at all. If you resize your window and try it again, you may see the problem. It might even have something to do with whether your browser window is an odd or even number of pixels in width, but I'm not 100% sure of that. But resizing the browser window seems to give you a 50/50 chance of seeing the problem.
The script is relatively simple: