How i can use background-image on container that not have content?

I have this structure:

<div id="container">
    <div id="header></div>
    <div id="content>
        <div id="sidebar"></div>
        <div id="main"></div>
     </div>
</div>

I need using background-image on container, but it not have any content, only header and sidebar, main have content.How i can do it without position: absolute or specify heigth in pixels?!