I modified the site settings to allow a 100x100 pixel custom avatar. And here's how I got my Xbox Avatar gamerpic in there...
- Go to: https://account.xbox.com/en-US/Profile?xr=mebarnav
- Right-click the giant picture of your avatar, choose "Copy Image Address"
- In a new tab/window, paste in the newly copied "ugly" URL for your avatar
- Change the very end of the URL from format=png&h=640&w=640 to format=png&h=100&w=100
- Now load that modified URL, and save your new 100x100 PNG image of your avatar.
- Go edit your profile
- Click on the "Avatar" tab
- Upload an Avatar from your computer (click to browse/select the file)
- Click "Save"