It is not really documented how you change this, but here is a short guide.
First you go to Settings->Media and upload the new image you want to use (for example mylogo.jpg)
Then you go to the web GUI home item. It is called "HomeWEB" and you will find it under All Items->GUI
In the HomeWEB item you set the "RightBanner" to /media/<name of your image>, for example /media/mylogo.jpg