I would reccommend:
1 upload your pic / banner to your geocache (rechte Seite Funktionen für den owner)
2 use within your listing html coding like
<p align="center"><a href="http://coord.info/GCxxx" ><img alt="GCxxx - Cachename" src="https://link zum Banner" height="200" /></a></center></p>
Link to the picture can be an external photo storage website or the link to your uploaded picture (rechte maustaste Link Bildadresse kopieren)
If you asked for creating the pic/banner, I would reccomend user a pic orr foto, preparing a rectangle in powerpoint or paint. Design your banner perhaps with your GC Coden on it and the save the pic.