I think we should go with the mirror. It has a larger surface area than the frames, so I think it is a good option. Plus, it's flat which will make my mosaicing life easier.
Pam - The rim is three inches wide. Are you going to send me a digital file of the photo collage? Don't worry about it fitting into a grid because I probably won't put the tiles in a simple grid formation. I'll probably use round tiles and other shapes to make sure you can see the best parts of each photo. How are we with photos? I don't have any digital ones, but I can scan a couple if you need more.