Not everyone is as familiar as a moderator with each forum's rules and software
It's not the forum's rules so much as it is the law. Posting copyrighted images by other people without permission
is illegal on any US based forum. (This forum is hosted out of Minneapolis).
Yes, there is the
fuzzy realm of fair use, but hosts of a website are not going to want to sift through every image and decide whether or not they want to gamble on it being fair use or not.
I don't even know if this is your photo or not. If it is yours, then not an issue (and it's a cute photo, I like it, btw). If it isn't though, then it's reasonable to expect you to know better without even having read forum rules.