http://www.moggies.co.uk/warning.html#photo I knew I'd read somewhere that the chemicals can be hazardous to cats. Here's what one site says...

Cats Licking Photographs

Your cat can suffer severe kidney and liver failure after licking photographs. According to my local Veterinary Surgeon, the chemicals on the paper act much the same way as anti-freeze does to the feline body. So, please keep ALL photographs out of your cat's reach.
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Why do cats lick photos?
Feline behaviorists believe cats find the odor of plastic irresistible. The cool texture feels good on a kitty's tongue. Also, photo surfaces can contain propylene glycol (as in anti-freeze), which also has a tempting sweet taste. Needless to say, licking is not good for your pictures, much less your cats.