you should go to your profile (by clicking on the 'U' next to your post)
then point at your profile picture (in this case the U again), it should show the word 'edit' click on that and it should take you to a page to edit your avatar, including uploading a new picture.
Note there may be a minimum number of posts you have to make before you can do that... I expect it might be 5 or 10, so you may just need to make posts more in the general forums (not GCC etc.)