Adding a Banner ad to the head of the site?

Hi all, good community here!

I am wondering if anyone has any tips on adding a banner ad to the head of the site.

I'm currently using 1.7.1 without a theme, and would like to have a banner (or two side-by-side) just beneath the site name and above the user menu and the "who is online" plugin I added.

I'm assuming this would be much like adjusting the spotlight and footer?

I am more nervous about changing this than I am the footer, as it is a live and active site. If anyone can point me in the right direction as to what file I need to edit and what I would need to add where in that file, or could point me in the right direction to find info, it would help me a lot.

And, I know Team Webgalli (sp) sells a plugin for ads, but I'm not interested in adding Adsense and I specifically want them in the top of the page (which I don't think is an option in that plugin). Thanks :-)