I accidentally uploaded sensitive images (containing a name and address) to a different Discourse forum recently, and I’ve heard that it’s easy to find such images simply by changing a url.
I’d like to request that this image is removed, but I don’t know whether I should contact that forum’s admins or if I should contact Discourse officially?
You should contact the forum you uploaded the images onto admin(s). Discourse only provides hosting (to customers) / the open source code for self hosters. It doesn’t manage forums.