I am guessing here, so please correct me if I am wrong (read: when I am inevitably wrong). En tavla is something that has a frame, including 'a blackboard', whereas en målning is canvas, or something similar, with paint on it.
I’ve never thought about it that way but it sounds correct. Note that en målning can have a frame and still be a målning however, it doesn’t have to be either or. It can be both.