I'm not sure what you mean, if your talking about a avatar which is right where your information is (post count, rank,etc...) , to select one, go to "My Controls" and click on
"Edit Avatar Settings" there you will be able to view a few choices, and set one as your avatar, so from now on under your name there will be a image of your choice.
IF your talking about how to put a picture directly on your post, for example: Like having a picture right here in this area, then you can't, only the Administrator "Admin" can do that.
Hope i helped.