A forum has specific guidelines (or restrictions in some cases) on subject matter. For example, on this forum we discuss community management, and we wouldn’t talk about celebrities or model trains here. Generally people ask questions and other people answer, and the primary goal is the exchange of information. With social media, the primary goal is to be social with other people, so even though the mechanism is very similar, the discussions can and often do vary widely. However, that is an arbitrary and very general guideline.
I would advise that it is much less important to define what your forum is or isn’t, and more important to just make sure it fulfills the needs of the community. If it helps people to be able to upload photos, then enabling it is a good thing.
hope that helps!