[@Eclektik] He needs to have at least a fundamental understanding of the creatures attributes and capabilities, and the photos have to be fairly well detailed, not a sketch or just a part of the creature. A well rendered depiction of an extinct creature could also be used, provided it's physically accurate to biology. If there's anatomical diagrams or he can study it in person even better, that makes it easier for him to change into that animal. He'd also have no way of changing into a creature that's just been described to him, no matter how detailed it is.