Admincp>Advertising>Add new ad> Select the field you want to display your ad> where it says Ad html> You just have to insert the the html code.... Sounds like you just want to display a .gif as a link on your page. to do this is simple.
Upload your picture to the root of your forum (or just upload it to tiny pic if your not that inclined yet) then make it a link using html. Use the code below but insert your personal information. and save.
<A HREF="The URL of the site"><img src="The URL of the image"></A>
Leave the quotes and only edit whats in between them.