It's similar to an emoticon, or 'smiley'.
It's called a facepalm, link -->
http://en.wikipedia.org/wiki/Facepalm
and I used it to express my disappointment with your post.
Mostly because the OP came here with a question and your answer implied that he
should learn to figure it out for himself.
The facepalm (and other 'GIFS') are mostly used in a lighthearted, humorous way, and my explanation is just to help you understand why it was used, not to offend you or start an argument. We all have different opinions
and that's what makes places like this fun. I was just trying to insert a bit of humor.