You can add replace deafult Advertisement with yours, Go To Admin > Advertise Settings

For adding text link

<div align="center">
<a href="YOUR LINK HERE">Text Here</a>
</div>

For adding image with link

<div align="center">
<a href="YOUR LINK HERE"><img src="http://www.tube.onlinehumtum.com/templates/images/only1internet.gif" border="0" alt=""></a>
</div>