Humor is subjective to the one viewing it. Thus, even if someone thinks that something he or she is posting is funny, that does not guarantee that the one viewing it will also think that it is funny. A lot of times single images posted mock the "mannerisms" of certain users in topics which can be construed as trolling - usually when others start posting images after that then it just devolves into a spam topic. There are few cases when one should just post an image and generally it should be avoided.