My avatar picture defaulted to, I think it's the photo of the guy from the SCV unit. How do I change this to something else? I can't seem to find an option for this anywhere.
Click on your photo in the upper-right to go to your profile. Then, click on the Rewards menu item, and then Portraits. You can see all of your choices there. You unlock more choices through achievements.
You cannot upload any photo you want.
The quickest way I know is to right click on your portrait in the top right corner and choose "Change portrait".