For me, I don't comment on pictures unless I feel that I can contribute something to the artist in an effort to help them out with the image or with future art.  When I do that, I also make an effort to point out what I like about the pic too.

I always try to remain constructive.  Also, I try to maintain a "if you can't say something nice..." attitude.  I know what damage a harsh critique or nasty one can bring.