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That's just a repeating background on the container...i'm not quite sure what you're asking and mean by "flexible".
 
Yeah like manueld said, you need to make an image that is the full width of what you want, set it as the background of your div, and make it repeat vertically (meaning, along the Y axis).

Your CSS will look something like:

#container {background-image: url(images/bkgd.jpg); background-position: 0px 0px; background-repeat: repeat-y; width: 900px; margin: 0 auto;}
 
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