I really like what you've got going here CA. I agree.
I'm going to throw out some good idea, but optional guidelines.
Have a picture for every 4 pages of text. If you have more, that's ok especially if they're small/informative example pictures. You can stretch the number of pages between pictures a bit, but don't go too far.
Have a picture for chapter headings. Ideally, you want something that exemplifies the chapter. You don't have to do this, but it's a good idea.