The answers given to Julia on http://www.italki.com/question/267199 are worth looking at, but yours is a more specific question than hers. Julia's question centred on the more general meanings of 'think of' meaning to have something enter your mind as opposed to 'think about' meaning 'consider'. In your example, the two expressions are actually very similar. Here, both 'think of' and 'think about' refer to an opinion. And yes, you are on the right track when you say that 'What do you think of the picture?' means basically 'Do you think it's good or bad?', while 'What do you think about the picture?' is asking for a much broader opinion. It's quite a subtle distinction, though.

I once asked the same question and these are the answers http://www.italki.com/question/267199 I think they are really helpful.

To be honest, I would give the same answer to both questions. The only thing I'd say is that "think of" sounds marginally more formal to me.