cant go back to layer(s) after they clicked

[Guest_imported]

I have a site where there are a bunch of thumbnails at the bottum of the page, and when the user clicks on them(or mouseover) the full image shows up above all the thumbnails, then when you click(or mousover) on the next thumbnail the next image takes the place of the previous, which is right, but when I try and go back to a previous thumbnail and bring it up, the current image stays up there. I'm using layers, and I checked 'Prevent Overlap' How do I make it so that I can go back and forth either way with the images? The site is online if you want to look

http://www.freshink.net/permanent/temp.html

Thanks