1. Has the material you have taken from the original work been transformed by adding new expression or meaning?
2. Was value added to the original by creating new information, new aesthetics, new insights, and understandings?
If it is parody - and this certainly is - then it is given broad latitude. And then there is the issue that Zimmerman is engaging in political speech which is the most hallowed of protected activities.
Like Warhol's Campbell Soup painting, Zimmerman's work is certainly transformative.
I won't enter the debate on artistic merit. It is irrelevant to the discussion at hand.
Granted, an exact copy or reproduction of the AP photograph would seem likely to infringe their copyright, but a work which is based on a gesture made by someone in public, and recorded simultaneously by many people seems to be a much more uncertain issue. Particularly when it may well be a commentary on the media, and the biases of AP itself.