I am building the under construction http://www.canprint.org
forum. I am intrigued by your confirmation screen, and may try to implement something like it. However, I am using SMF 2.0 rather than phpBB 3.0.x, so it may be difficult. I'm going to put in a support question to them to check.
(isn't there always a "but" ...)
I had the impression the confirmation was a trick question, and had trouble actually getting confirmed. Some of the car pictures had model names of animals, like Vipers (that one was obvious), perhaps Jaguars, probably Mustangs, etc. However, the pictures were of low enough resolution that it was difficult to tell with some of them.
And I'm something of a "car guy".
Perhaps that is "as intended"?
I have had huge problems with spammers on another forum, so it is a tradeoff to reduce their presence.
I can relate to not making it obvious to register, especially for bots. I just registered recently on a forum that asked questions in French, but they were obvious if you scratched your head ... kind of like:
"Who is buried in Grant's tomb?" in French.
HTH. From what I can tell, you have a very impression piece of software.