I haven't read all the posts so some may have already been mentioned, or done before (I'm new obviously):
Funniest Adult doing something very childlike photo (i.e.: old man wearing Mickey ears trying to pull out the Sword in the Stone)
Best "rare" photo: one that would be difficult or impossible for anyone to reproduce, like some behind the scenes stuff, a ride breakdown, catching a cast member picking their nose or something ( ) Obviously not something that would make Disney look bad or anything, just something....rare. Use your imagination....I have a couple
Most clever pose with a prop/cast member/character (example: I have one of me that I love under one of those "Mickey's hand pointing down" signs, looking up at the sign and pointing to myself like, "who, me?" My caption in my scrapbook says "the mighty hand of :mickey: chooses his followers!") and I have some silly ones with the cigar store Indian. (of course, nothing lewd allowed so you'd have to get creative!!!)
Best cameo of a facial expression of someone (kids would be great but so would adults!) reacting to something at Disney, like a look of surprise or terror, or laughter or happy tears....you get the point (like the guys on the pole at Jungle Cruise! hardy har).