Actually, Ditto has been seen to transform into pictures and even extrapolate forms that don't exist from pictures, so what you're suggesting should be fine, especially in light of Beast Boy's Enlightened abilities coming into play as well. We propose that Beast Boy will be able to transform into anything that he's seen before in person, or has seen an accurate picture representation of. However, he will be unable to use special attacks, as you suggest, being restricted to things like biting and kicking. This is to prevent him from immediately getting the type advantage on anything he runs into.

You may pick a few forms that he is very familiar with and that he can use the special attacks of, though.