Thanks for the error pic, that looked vaguely familiar. Unfortunately upon heading to the phpbb support boards I got a message that they're recovering from a site breach and will be back up as soon as they can (comforting, I know).
At any rate, as soon as their support forums come back up I can research this further....I think it's vaguely related to a combination of:
a) Too many attempts/failures in a short amount of time
b) the fact that the "simpledark" theme pack I use was originally created for an older version of phpbb (the language that runs this forum), I had the pack modified when phpbb updated to version 3.0 but there might be a snipit of code missing that's not showing the CAPTCHA
Since I'm not sure exactly when the support forums are going to be back up, we'll just have to wait on this for a bit. In the mean time, if you host your pics anywhere (Imgur, flickr, photobucket, etc) you can just link photos instead of attaching them.