You are not at all alone. Many cat owners report the exact behavior exhibited by your cat in their own pets and they are just as baffled. Though the attraction to plastic varies in degree from individual to individual, it is not at all uncommon for cats to lick, chew, and even eat plastic.
Cats who seem to be most obsessed with plastic are often the type of feline who may still try to "nurse" as an adult (sucking on blankets, or their people's clothes, etc.) or are otherwise orally fixated. These are cats who were most likely taken or weaned from their mothers at too early an age.
One major concern relating to this behavior is that when cats consume plastic, especially the decorative plastic grasses that line Easter baskets and gifts, or plastic tinsel, they can become seriously ill from resulting intestinal blockages. Consequently, you should try to keep your cat from ingesting plastic.
Many plastic products today are made with biodegradable components, including cornstarch. These ingredients along with various petroleum products emanate an odor, undetected by people, but irresistible to certain felines.
The texture of plastic also seems to be a factor in this bizarre activity. It is speculated that cats just enjoy the smooth, cool feeling on their tongues. This combined with the aroma may give cats a very pleasurable sensation akin to enjoying a lollipop that never gets any smaller no matter how many times it gets licked.
I do caution you to keep all photographs safely stored away in photo albums or behind glass frames. Some photographic processes use chemicals that can potentially cause liver and kidney damage if consumed. So please be very careful with these items.
Licking or eating non-food items may also be a sign of a serious illness such as feline leukemia or other immunodeficiency diseases. Just to be on the safe side, you should have your cat tested. Most likely however, this behavior is nothing to worry about as long as you restrict it to licking and not consuming.
If you feel you must stop your cat from continuing in his plastic quest, you should first eliminate the type of plastic he targets from your household. Next, increase his fiber intake with specially formulated high fiber pet foods. You can also grow "kitty grass" indoors so that your little friend has something more natural to chew on or lick.
Finally, some cats become addicted to plastic licking simply because they are bored and don't get enough stimulation. I would strongly recommend getting some new toys for your cat and adding some extended playtime to his daily routine. Give him a lot of extra affection or brush him more frequently. If he is alone during the day and you work, perhaps you should consider getting him a little feline buddy with whom to pass the hours while you are away or busy.