I would start with the following ideas for decryption keys:
long alpha-numeric names of photos Q posted (ex. Dkrr0VeXgAArTN3 [.jpg] from 4914
repeatedly used phrases, trying upper/lower case variations, substituting . (dots) for spaces, underscores for spaces, etc.
-- WWG1WGA
-- You have more than you know
-- God Wins
-- These people are sick
-- etc.
I would start with the following ideas for decryption keys: