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You can either edit your theme template and put in an image link where you see a reference to the XOOPS banner display, or you can go to your admin panel under Banners and install your own graphics as banners. This can actually give you a clever graphic display since you could have a different version of your image come up each time the user refreshes his/her browser screen.
An example is here:
http://www.bloggerforum.comThe site logo is the same in each banner, but the person graphic changes at random with each screen refresh.