If you want to use a background image, the only way I could figure it out was to do it like this.

In the index.html file, add the following line right before "fo.write("flashcontent");"

fo.addParam("wmode", "transparent");

This will set the flash background to transparent.

Then, add the following style in the header of the index.html page with the URL of your background image, as follows:

<style type="text/css">
body
{
background-image: url("http://site/images/bg.gif");
background-repeat: no-repeat;
background-position: center center;
}
</style>

Then the flash is transparent and shows the background image of the body of the page.

@johnnylox: You are a god. I've been looking to add a non-repeating background forever, with no luck, until I read this post, and now I can do it! Viva you! :D