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If you use a theme that has a logo (I assume it's the default since there's a XOOPS logo) just look up the logo.jpg, .gif or wathever it is, in your theme folder and replace it with you own.
If you want it screenwide and still a banner, look in your theme (theme.html)and find where the background (colour or image) is defined, either in the table/td or in the theme.css. Raplace it with an url to your logo.
edit: Look
here for lots of info concerning theme/template modification.
I am the BEST.......... at overstating things! -