If you mean the one that appears at the top of the page on the left side next to the Unread messages and My Checklists links, you can't change that one. It's a permanent avatar that everyone on the site has.
If you mean the one that appears on the left side of the screen on your forum posts, you've already edited that one so that answers that
There's also an avatar that appears on your Profile page. To find this avatar click on your PS3T user name at the top of the page on the right side when you first log in. You can also find it by clicking on the User CP tab at the top of the forums page. From there, you can select the Edit Your Profile Picture link on the left side and you can select a picture from your computer's hard drive or provide a web address to the picture you want to use. Just make sure that it is appropriate for use here (no nudity or porn, isn't obviously offensive, etc)
I hope this helps mate. Feel free to ask if there's anything else you need
Cheers