This article might help: http://support.citrixonline.com/en_US/training/all_files/G2T020020 The key in this article is "Note: If you do not see the Branding tab on your Settings page, then you are part of a multi-user account and these settings are managed for the entire account by your administrator(s)."
I thought I was the admin, but discovered I wasn't. Once that was sorted out, I still couldn't add branding via the GoToTraining page, like the above article suggests. I had to go to the "admin center" using our admin's login credentials and access the "admin settings" in the left menu bar. Once you find that, it's pretty intuitive. Hope this helps, and good luck!