I haven't seen anyone getting banned for posting images of others here on the forum. Typically, there have only been situations in art contests where people claim artwork that isn't theirs, resulting in disqualification. While it's not a ban, it's still a form of plagiarism because if you didn't create the image or content yourself, you should give credit to the original creator or reference. I usually prefer adding a link below my images to show where I found them. It puts my mind at ease of not being a plagariser.