People throw images at our faces, so why can not one throw words back? Putting images out there to be seen is a form of communication -- and once communication has started, why does one party get to determine the course of the communication?
If one is willing to accept positive comments, one should also be willing to take negative comments. People making comments should also be willing to accepts critiques of their comments.
But due to the need of politeness and brevity on this forum, I can see restricting critiques of posted work here, unless critiques are invited/requested by the poster. Without the permission to freely critique, people just have to realize that all the positive statements about one's work are nice ego-strokes, but should not be taken as true critiques...and that the work may not be free of glaring problems.