Banners on Website

That would depend, and actually a good question.
If you use a javascript that calls up rotating banners,
then that is what the html code will show. Just a
line of code calling the javascript. It's true that google
does run some javascripts, but not always. And running
a javascript does not mean they incorporate such
stuff when evaluating your site for SERPs.

You would want to make the text and links as hidden
to the googlebot as possible. Using a pure image
would certainly help, no alt text and no text link underneath
or associated with it.

The reason is simple. If you have a link high up on a site,
it could skew what google thinks the site is about. Everything
you add to a page reconfigures the page. Sometimes it
has no effect, sometimes it does. The PR of any other
links on the page will be affected.

Also, you do not want any link or info on the page saying
that you are selling links. Just have a simple link to
a page that says to contact you if you wish to advertise.

Some people will say it's no big deal, but any little tweak
to a website can have devastating effects.

I would use a javscript to call the banner, and use a
url shortener for any url.

You should only allow people to buy ads on your site, not
links for PR or search engine benefit. Or at least make it
seem like you are not doing that to google.

A banner on the bottom of page would be a moot point.
Very low level link to begin with.

Paul