First of all, to create your own advertisement banner - you should create a banner picture using an image editor (for ex: Photoshop).
Then you should share it on the Internet. Or you can upload the image to your network via Admin Area > Edit Theme > Graphics, and then use the following banner code by adding the link you received after uploading:
Then click on the plugin settings button which appear after you hover of the plugin and add a banner
<a href="YOUR_SITE_URL"><img src="URL_RECEIVED_AFTER_UPLOADING" border=0 alt="SITE_TITLE" /></a>
 
To create a simple link - you can use the following code:
 
<a href="YOUR_SITE_URL">THE_LINK_TITLE</a>

Then place the banner to the pages you like in the Banner Placement section