Go to your my ebay or your feedback page and click on your id at the top. That will take you to your profile page. Click on edit profile and then you should see a little symbol on the top corner of the picture area. Click on that and then on change pic.
The pic that you upload to that page will show on this forum.
I might be a bit off on my instructions as I uploaded my pic a long time ago but that is definitely the page that you load it to.