I agree that it's a good picture. But "I took a great picture of a monkey" doesn't sell nearly as well as "This monkey took a great picture of himself". "I took a great picture of a monkey" doesn't get you articles about that picture, "This monkey took a great picture of himself" definitely does.

Yes it is a great picture, but it would not be not worth anything (I mean economically, not artistically) if it was not taken by the monkey.