Go to your domain edit page.
Scroll down to bottom of that page.
Click on square button next to "Custom Content".
This will open up custom content editor window.
To add affiliate banner...
Click on menu icon that look like computer monitor on top.
This will give you popup window.
Type in your URL for your banner image.
Click "Insert" button.
Now, you should see banner image.
Click the banner one time.
Click menu icon that look like chain (unbroken chain) on top.
Type in your affiliate URL.
Click "Insert" button.
Here is one of my site with banner.
It's not affiliate banner, but, it should work same.
http://www.lensfitter.com/ 
