The newest zenphoto has issues with High res images.
I run a texture site with 22MP images and recently upgraded to find most of my thumbs were gone.

To fix this I downgraded to 1.1.7 and everything worked great.

It took me awhile in beating 1.5.2 to death trying to figure out the problem which has alot to do with PHP memory

1.1.7 cares less about memory therefore it works great.

Keep it simple keep it Zen

That actually has only to do with php memory. You did read this?: http://www.zenphoto.org/2009/03/troubleshooting-zenphoto/#11

Also there are other reasons possible why you did not get thumbs. But of course you did not tell any details or errors you got...