HGG Community Forums

They are "horse headers", and I believe you need to have a basic or premium upgrade to use them. If you are upgraded, then you just go to your "Account and Stable Management" tab on the home page, then to "Manage Stable Settings", and there is a button for adding a horse header. There are specifications on dimensions and file size, but you can use any image of the correct size, hosted on an image hosting site like imgur.

Edit: Actually you don't have to host the image for the horse header. I was wrong about that. It is a direct file upload :)