I come late but here's the code that stands in the external 
*.css-file:

body {background-image:url(path_to_image.gif);}

If the style is defined inline, it says for example:

<td style="background-image:url(path_to_image.gif)">

I just downloaded Version 3.20RC7, and I like very much the 
link creation feature for dynamic pages. It makes the 
software great for creating presentation copies. Only these 
background images are missing (well, it is not very much 
work to copy them into the folders myself, by the way).

Thanks Markus