Ok, you have a few issues here which are not helping the situation:
a) Your basic HTML document structure isn't quite right. There's info here to get you started:
The HTML Document Structure
b) You have HTML and XHTML coding mixed together. Pick one style and keep to it. You define which standard you are coding to by
declaring a DOCTYPE at the beginning of the file.
Differences Between XHTML And HTML
c) You managed to get 2 opening body tags in your file. You'll have to delete one.
These just a few things I noticed when viewing the source code for the page.
I would suggest starting over instead of trying to fix this:
a) Create an external stylesheet for the site. This will make it much easier to make changes site wide and will help clean up your code (= faster load time).
b) Create a template to be recycled throughout the site. When you want to create a new page, all you have to do is open the template, save it as the new file name then start adding the content for the page.
c) Test the template as you add features. This way it is easier to narrow down problems if you test each time you change something or add something.
d) Use the W3C validator to check your coding as you go. This again will make it easier to track down problems.
Once you've got the template done and validated, then try adding the banner script to a new page created from the template. This way you eliminate the basic page coding as the problem and can focus on troubleshooting the actual script.
I'd also suggest that you:
a) Have a folder for your images (images/imagename.ext would then be the path to the image)
b) Have a folder for your css files (css/filename.css would then be the path to the css file)
c) Have a folder for your scripts (scripts/filename.js would then be the path to the javascript file)
The above suggestions would help keep your webspace organized and also, you can set it up to keep the search engines from indexing these files.
Post again if you have cleaned up the coding and are still having problems with the script.