A lot of artists, professional and amateur, have gotten into legal and financial trouble by using reference photos without permission of the photographer. Copyright law states that a photographer has the rights to his own photographic images. They can and do sue if you use the image without getting permission and/or paying for the use. This is not specific to Zimmerman, though his own fame no doubt drew the attention of the AP photographer in a way a person who wasn't a public figure might not have done.
No joke, I know a woman who painted a picture of a horse and submitted it to a contest. Someone else suggested that she had used his photo as her reference. The contest organizers correctly made her prove that she had used a different reference photo as the basis of her painting and had gotten the legal right to it.
It would seem that the copyright of the image belongs to the horse (even in the case of Corey).
http://en.wikipedia.org/wiki/Rogers_v._Koons
Rogers v. Koons, 960 F.2d 301 (2d Cir. 1992), is a leading U.S. court case on copyright, dealing with the fair use defense for parody. The United States Court of Appeals for the Second Circuit found that an artist copying a photograph could be liable for infringement when there was no clear need to imitate the photograph for parody.
Art Rogers, a professional photographer, took a black-and-white photo of a man and a woman with their arms full of puppies. The photograph was simply entitled, Puppies, and was used on greeting cards and other generic merchandise.
Jeff Koons, an internationally known artist, found the picture on a postcard and wanted to make a sculpture based on the photograph for an art show on the theme of banality of everyday items. After removing the copyright label from the postcard, he gave it to his assistants with instructions on how to model the sculpture. He asked that as much detail be copied as possible, though the puppies were to be made blue, their noses exaggerated, and flowers to be added to the hair of the man and woman.
The sculpture, entitled, String of Puppies, became a success. Koons sold three of them for a total of $367,000.
Upon discovering that his picture had been copied, Rogers sued Koons and the Sonnabend Gallery for copyright infringement. Koons admitted to having copied the image intentionally, but attempted to claim fair use by parody.
The Court found both “substantial similarity” and that Koons had access to the picture. The similarity was so close that the average lay person would recognize the copying, a measure for evaluation. Thus the sculpture was found to be a copy of the work by Rogers.
On the issue of fair use, the court rejected the parody argument, as Koons could have constructed his parody of that general type of art without copying Rogers’ specific work. That is, Koons was not commenting on Rogers’ work specifically, and so his copying of that work did not fall under the fair use exception.