There is also the issue of consent—it's rude at best, and an invasive violation of their privacy at worst, to post a picture of someone if they aren't on this forum and don't give the OK. Obviously, if you post a picture of yourself, you can consent to posting that, but a picture of yourself and your friends is not because we don't know your friends and have no way of knowing if they're cool with their picture being posted here. (We also have no way of knowing whether a photo you post here of "you" is really you, or some poor guy you're pretending to be.)
More generally, it's that irrelevant posts are not allowed on the forum and photos of people are rarely relevant. We don't know your friends, and if they're not roadgeeks, we don't really care about them. Personally, and I grant that I may be in the minority on this one, but I don't care what anyone's face looks like at all; I get to know members on here by their username and what they post about, so it doesn't help me at all to know what their face looks like unless I'm trying to meet up with them a road meet or something. So people wanting to share a picture of their face feels weirdly intimate, I guess.