How do I change my profile / avatar picture on my account for Starcraft II?

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.

Solution 1:

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.

Solution 2:

The old Rewards section in the profile dialog has been replaced with a new Collections tab in the main menu. The Portraits sub-tab allows you to select any portrait you own, which you can activate by pressing the Equip button.