MouseOver & Image swap

Ok, here's my situation.

I have a page that loads. During that time a product image is loaded
into a <div> at the top of the page. As the page continues to load,
product information, including alternate product colors loads (as div's
with colored backgrounds).

What I would like to do is at a control that when a user hovers over the
alternate colors, a image swap with the original image that loads
into the div at the top of the page with the color that is moused over.

I guess my question is how do I get the new image to appear in the spot
where the original image was loaded. Also, can I assign this action to
a div tag with nothing in it? Just a colored background?