It's a great picture. I find myself cycling between the two 'main' heads as neither of them is sufficiently emphasised as to count as the focus of the image but actually I think that is part of what makes the image work. The pup on the right keeps pulling my attention which makes me smile then I go back to looking at the other two heads. The end result is a dynamic image which keeps the eye moving and so doesn't get boring.
It kind of breaks the rules but it works despite (or perhaps because of) that.
