OK, well, that's different, you are looking for something like an ajax, hidden div tag which reviels itself on the mousover of another?

Even though you will be using images, think of it more like a customized tooltip.

There is actually a javascript library located at http://blog.innerewut.de/tooltip
which I have used in the past. All that is required is one small piece of code.
Download tooltip.js I believe it may also require prototype.js (not exactly sure about that since it's been a while since I've used it).

Then all you need to do is include them in your head tag, create the two div tags in your body (one hidden, one not), then at the bottom of your page

Replace visible_div_id and hidden_div_id with the id of the coresponding div tags.

What you need to know though is that your visible div tag must have widths, otherwise the trigger will happen an unwanted places.