We always used rubber cement to mount pictures to matting when I took photography. It worked well although it does not protect the fronts of the pictures, if that's what you're trying to do.
Thanks for the suggestions! I had never thought of the balsa wood...I like that idea. Just have to check which looks best and is most cost efficient.
I was thinking of using it as a seasonal/halloween decoration...putting winter pictures in the winter, Halloween pictures for Halloween, etc. So I want to have a bunch to choose from.