When linking to affiliates, its usual that they will provide an image for you to use. But if you would like to create one then all thats needed is an application like photoshop, size being 88x31 and go from there. In most cases, its a slick effect with a pixel font of the sites name. Although the creation needs no html knowledge, as ou may guess, getting it onto a page does, and can be done with the following code:
src attribute being where the file is on your server. Hope this helps.
<a href="http://www.thesiteyouarelinkingto.com" target="_blank"><img src="images/thesitesimage" width="88" height="31" border="0"></a>