I'm working on a portfolio site, it has a gallery which is fixed up to a CMS. Here's the set up all the images are to be placed one on top of the other and then the thumbnails are actually masked portions of the image, when you hover over one the image is shown in place of all the other 'masked images' - a simple fade in fade out with JQuery. The gallery is like a series of preview boxes (the masks) just around 70x70px pretty small here's an example of how it would work:

http://imgur.com/t3lJa

That said, I'm having trouble with masking these images because since it's going to be run by a CMS so the actual locations of the masks aren't going to be the same so it'll be like a moving mask as newer images are uploaded. I've tried using the clip property in the CSS but that needs for the mask to be placed in one spot hence not an option. Any ideas I'm stuck :/