People who post art online really need to remember a saying that one of my Animation tudors said "If you don't want someone pointing out things that are wrong with your art don't put it online"
It's the internet people WILL be harsh it's just how it is. You just need to learn to either accept what's wrong and use it to get better or ignore them. Don't act like a child and snap back, that wont do you any good.
This is extremely important if you ever want to get a job doing art. You wont last long if you snap at your boss
You can't expect to get nice comments all your life after all!
Sorry it's just such a pet peeve of mine. I've been seeing more and more people I watch act like children when it comes to this and I lose respect for them when I see it.