To some religious zealots, even showing part of your feet is offensive. Everything to them is pornographic.
It's so subjective. Different people get aroused by different things. Some images are explicit, others are implicit, like when you view a scenery of twin peaks and a jungle below... well, you know what lah...
For most others, the simplest and most direct answer to "what is the difference between pornography and erotic photography" is:
"a very fine line".