Photo Gallery rollover image Image Swap

I am using the XML Photo Gallery with thumbnails, and i have managed to modify it to suit my needs for the most part. However, some of the work i want to use it to display is photo restoration and i would like to be able to rollover the image and have a different image show on rollover. Something like you see here…http://homepage.mac.com/gapodaca/digital/bikini/ only using the XML Photo Gallery.

Any ideas?