Sadly even with terms and contracts, people still steal images. It happens to us all the time. We've had people print construction billboards with our watermarked renderings. Watermarks are annoying - but make sure they are clearly across the entire image. We watermark our entire online portfolio now, like this: http://www.studio2a.net/portfolio
In this case there isn't much you can do. This is why rule #2 - "Don't do stuff for free". You could spend a lot of money on a lawyer to send them letters to try to get compensated. I'm guessing you want to stay friendly with this architect and developer? You could ask them nicely to not publish them, but in the end maybe just chalk it up to a learning experience.