Today I am added the Logo Change Mod for Default theme.
You just need to find a place to upload your banner then just put. Maybe I will make an upload system for your banners in the future.
This can be found in your admin panel under current theme settings for the default.
I will also try add have this option added to other themes.
Edit: Added also added logo mod for the Classic, Salava, BlackDay, DarkRed themes.  Nigtfell,StarFell and SMFOne already had the logo mod built in.

Enjoy,
SMF For Free