I was working on a project and was looking for more control over the elements I was creating in the HTML.  More specifically, I had a MasterPage setup and I wanted all pages to have the same background image throughout the site.

Simple.  Add a runat=server to your markup.  See below. 

<div id="bgimg" runat="server">
</div>

You now have control of this element from the code-behind.

To add a background image to the div, you could do something like this:

bgimg.Style[HtmlTextWriterStyle.BackgroundImage] = ResolveUrl("~/images/bodyback.jpg");

The only thing to realize is what your generated markup is.  Asp.Net will auto-generate this tag to prevent naming collisions.  Just look out for it if you are trying to style the element with CSS or call it using javascript.

<div id="ctl00_bgimg" style="background-image:url(/HRR/images/bodyback.jpg);">
</div>

Enjoy!