I had a cat that lived to be 18 - used to lick black and white as well as color photographs. The chemicals are not good for the cat, but it didn't seem to bother Demian since he lived to be 18. He would climb shelves and take them out of frames, just to get to them. Needless to say, he destroyed many a photograph until I learned to hide them.

I would suggest you try to discourage your critters from licking them. Maybe it is a habit - a certain smell? Beats me. No other cats of mine have ever done it.

Cats are odd little things, aren't they?