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Hi,
not shure what you want to do - you have to give some more information, but:
xoops has a build in banner management - go to Administration menu -> System admin -> Banners
There you can add clients, banners and set the imapresions. If you need more flexibility you may try a module called RW Banner.
If you changed your theme (the place where the banner is displayed) but the change does not take effect, you need to clear the cache - delete all files except index.html in templates_c folder.