The problem is there are people who say they want criticism so they can improve, then get bent out of shape if you take them at their word and offer it.

Back when I posted in the gallery a lot, I found that it helps if you respond to criticism publically, in your comments section. ("Thanks for the suggestion, I hadn't thought of that" or "I tried to do that, but it looked funny, because...") Sometimes I incorporate the suggestions and post an updated version. If people see that you actually do welcome criticism, they are more likely to give it to you.