My guess is that new sleeper accounts are easy to create. A dedicated freak can have dozens (I've actually seen this), with new ones routinely being formed. Perhaps the solution, not a great one, is to prevent a new account from posting pics until it's got a history of posting, say, 100 posts, maybe even 300. When the magic number is reached, the account automatically becomes "pic enabled." I can't imagine how to program that, but it's probably doable. Tough on legitimate newbies, but maybe necessary.
About I year ago I helped uncover a troll that had several accounts. They were all created on the same day and they were all in neat alphabetical order: "ablechair" (aka, "boltCutter", aka, "kodaChrome")