From what I recall, it's to stop the possibility of someone posting something triggering. Some people will feel they need to post an image of SH, or other triggering images. While the act might be innocent by the Original Poster, the effect can be quite damaging to people in a vulnerable state.
That and that detour people from posting personal information. Such as personal photos. I once used a picture of myself as my avatar. I pleasantly received a nice pm requesting I take it down because some user thought I looked like their Abuser.
Good times.