Your profile picture?
Did you try to post an image hosted on another site?
I'm guessing it is your avatar you're referring to. You can upload it by going to the User CP, click on Edit Avatar (it's in the column to the left) and upload an image being mindful of size restrictions.
I've been trying to change my avatar for the last 30 mins.. Can't get it to work.
I've tried different sizes, different formats and different methods (image hosting), different pictures..
Any one know whats what?