When distinguishing erotic photographs from pornographic photographs, we get into circular arguments because the definitions of each overlap.
Definition of EROTIC:
1: of, devoted to, or tending to arouse sexual love or desire <erotic art>
2: strongly marked or affected by sexual desire
Definition of PORNOGRAPHY
1: the depiction of erotic behavior (as in pictures or writing) intended to cause sexual excitement
2: material (as books or a photograph) that depicts erotic behavior and is intended to cause sexual excitement
I hope no one is offended by me offering definitions, after all, this is the philosophy section. I think it helps to start from common ground (defined terms) for productive discussion. Otherwise, we devolve into Alice in Wonderland where terms mean only what one means them to mean and nothing more.
If we start with those definitions, then the difference is intent--the intent of the creator. The images in the gallery were not pornography because the photographer did not intend to cause sexual excitement.
The problem arises, however, in that people are sexually excited by different things. The viewers tend to say something is pornography if (1) they don't really know the definition of pornography and hence anything containing sexuality in it is pornographic; or (2) the viewer is sexually excited by the image and therefore it had to be intended to cause sexual excitement.
The difference between "tending to arouse sexual love or desire" (erotic) and "intended to cause sexual excitement" (pornography) is a pretty fine line. If one puts the two definitions together, we end up with; pornography: the depiction of behavior devoted to, or tending to arouse sexual love or desire and is intended to cause sexual excitement.
There, that clears things up nicely.