First of all, 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:
<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>
So you need to insert this code: <a href="YOUR_SITE_URL"><img src="URL_RECEIVED_AFTER_UPLOADING" border=0 alt="SITE_TITLE" /></a> in the advertisement plugin
go the installed plugins, hover over advertisement plugin, click on the settings then add a banner, paste this:
<a href="YOUR_SITE_URL"><img src="URL_RECEIVED_AFTER_UPLOADING" border=0 alt="SITE_TITLE" /></a> in the code area and click add
then you need to place the banner at the footer,
then go to : www.domain.com/index/customize
add custom text/html widget to the sidebar, go the settings of this widget and insert the same code <a href="YOUR_SITE_URL"><img src="URL_RECEIVED_AFTER_UPLOADING" border=0 alt="SITE_TITLE" /></a> in the content area and save it.
I hope I made it clear for you.