Sooooo, it looks like somebody's hi-jacked my domain (or something) and that's why some people are experiencing issues registering... basically, google (and others I'm sure) are blocking emails from this machine due to a "large amount of unsolicited mail".
I'm currently working on this, but if you need support, you can always file tickets on github: http://github.com/masterexploder/PHPThumb/issues
You may need to create a github account to do so.
Sorry for the inconvenience this causes. I'm also highly considering shutting down these forums as I really don't care to, or want to deal with any kind of spam-related headaches.
UPDATE: Figured out what the problem was, so emails might start to trickle out soon. Give it a few days, and I'd imagine registration will start working again.