I haven't seen any guidelines limiting pictures, but I have had some deleted by editors. The audacity! :-)

I was a bit picture mad when I started here, and was dropping pictures everywhere. Then when editors started taking a few out, I realised I was over-doing it. Now I often only use one.

I think I'm frightened of losing the reader at the break where the picture goes, which is probably stupid. If they're bored they'll stop anyway; if they're not, they'll keep reading.