I think some of the bot owners may register themselves, or have found a way of hacking the database/scripts, though PHP isn't that easy...
But, we could always distort the image more, by flipping it, or curving edges, etc. (AFAIK, this would make it near impossible to be hacked in this method - bot's aren't clever )
Actualy, this is something I see alot of in phpbb2 boards... I've run several, and seem most of them evetunally degenerate into nothing but lists of spam. What gets me is how they're only targeting the General Talk thread. Perhaps we could try renaming the forum to 'Talk' for a little while, to see if it makes a difference?