You must have (or create for yourself) image size 64x64 pixels. Then upload it to some site on internet. http://www.photobucket.com will suit just fine for that purpose; create account there and upload a picture in your space.
When you are done with that part, return to the forums and select GO > MY SPACE > PROFILE > View/Edit Complete Profile. Click on section Avatar/Picture URL (edit) and in empty box Provide Custom Avatar: copy/paste your picture link that you have uploaded to photobucket.com.
I hope you'll manage.