I have commented negatively on images before, but only to artists whom know me, or of me. I always give reason why I post negatively, not just "This sucks" or what have you.

I believe critical feedback is more valuable than "OMG YOU ROCK". It's how I grow personally. I get critical feedback all the time. Sometimes, it's nonsense. Other times, it's valid. Not taking it personally is the key.

At the end of the day, the art is for the artist. Someone I posted negatively commented to pointed that out to me. They are right of course. Our art is for ourselves.

Some people want criticism. Others don't. As I recall, the button that allows user feedback does say positive and negative. If people don't want honest feedback, they don't have to enable that option.