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The XOOPS banner management system is pretty basic, and doesn't let you distinguish between vertical and horizontal banners. At present your only real option is to use something like PHPAdsNew to manage your adds. This is a great module, but the learning curve on it is pretty step, and probably way overkill for what most people are wanting out of their banner system. I would be quite happy with the XOOPS system if it accounted for vertical banners, but no go unfortunately. You might try nagging a few developers and seeing if they can add this feature in, I sure as hell would be in support of it