I was in a writing community years ago and the way that group worked was - You had to write X number of critiques of people's stories before getting the right to post a story of your own...  but that wasn't all there was to it, there were rules too - when commenting on someone's work it wasn't enough to say I liked this or didn't like that, you had to state valid reasons why,,  and it didn't stop there - the language you used in critiquing someone's work had to be polite, respectful, without even a hint of rudeness or you were temporarily banned from critiquing or posting a story for a period of time, then the whole thing would start all over again...  

My only point of saying that is - giving someone negative feedback on an image they post is only useful when it is done in the right way and valid reasons are given to why a negative comment is being given...   since art is only in the eye of the beholder, it really doesn't make a lot of sense to just say it sucks without giving some reason or reasons why, and stating them in a respectful way..   That's the reason I never leave negative feedback on an image - I've always felt that if someone wants my ideas on how an image can be improved and they ask I would be happy to do so, but it would always only be done in the most polite fashion since ultimately my opinion or anyone else's for that matter is just that - only an opinion...    

~>M