If the mind wanders while we're chanting, is it OK to watch lectures, listen to Krishna bhajans and kirtans or try to bring a picture of Krishna to mind? What things should we not do while chanting?

While chanting we should focus on the sound of the holy name. The holy name is Krishna Himself, so everything is there when we are hearing sri nama. Certainly we can look at pictures to keep focused on Krishna's form, but it's not helpful to do other things while chanting japa.

Classes, bhajans, etc. should receive our full attention, and chanting japa should also get all our attention. It isn't possible to place full attention on 2 things at once. Anything that requires or demands our attention is not good company when chanting japa. The first business when chanting japa is to offer full attention to the sound of the holy name.

I hope this is helpful.
Sincerely
Laxmimoni dasi