Edit is a poor choice of words... it's actually a way to assign or upload a picture that you've already got the size you want... in this case a 125x125 pixels or less...
Go to Settings at the top right of the forum screen, then on the left menu find Edit Avatar... Select either a pic that you already have online or select to upload one from your computer and save when finished making your selection.
There is no place for me to upload a pic.
This is all I see:
Your Current Avatar
No Avatar Specified
Avatars are small graphics that are displayed under your username whenever you post.
Do not use an avatar
Note: if you have a custom avatar selecting this option will delete it.
As Matt stated, you will have the ability when you have 50 posts. At first, it's a small avatar, but the more posts you get, the bigger you can upload. Same with signature as well.