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The "sponsor" banner is just a placeholder for the banner to show you what it should be.
So if you have a banner that is smaller than the default 468 x 60, then it shows up underneath of it. Or if the link to the image is wrong.
So if the client image shows up, then the link is good, but the size might be wrong. So you'll need to either adjust the image size to 468 x 60, or go to the theme and comment out the line with the background image. For XOOPS themes, it is normally in the theme's style.css file, and you should comment it out:
background: url(../img/bg-ad-top.png) no-repeat;