How to put banners into a WP header???

[Element115]

I'm trying to embed a banner/or banners into my wordpress header and I can't seem to do it without completely screwing up the header itself, or the banner just doesn't align correctly. I've tried searching for a way to do this and can't find anything. Can anyone point me in the right direction or feel like explaining how this is done?

For reference, this is the blog I'm trying to do this on Skinny Teen. I'm trying to put the banner into the empty space on the right side. Any help with this would be greatly appreciated, it's driving me nuts!