I don't know of something that already does this... but I suppose you could use the personalization framework combined with a skin object to allow users to select from different "themes" that would be applied using CSS changes.

 I thought I already posted this, but you can use the page large icon for this.

Set it in the page settings and use this in your skin:

<div style="background: url(<%=PortalSettings.HomeDirectory & PortalSettings.ActiveTab.IconFileLarge %>); height: 100px;"></div>

I think you should remove the  "PortalSettings.HomeDirectory" part for DNN 6+

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If it helps at all... this is (probably) a better case for a skin object. That way, you can just put the skin object in your skin file and have it take care of this for you. With a module, you'd have to manually add it to each page (which would be terrible), or do the "show on all pages", which is also kind of a mess.

Hope this helps,

Mike