i promised a Xara solution.
Golly, I made it: Xara - Hover Up Images.xar

No code, just a clear head.
The good thing is you can add the full range of Xara animations, times and delays.

Do note it is a proof of concept and you need to work on it for your needs. I would, for a change, like to see your examples and explanation of any problems rather that having to do it all for you.

Acorn