A 1px left padding has been applied to the body and if you now resize the page you will see that there is no jog between the header and the body background image.
<!--[if IE ]>
<style type="text/css">
body {padding-left:1px;}/* for ie only*/
</style>
<![endif]-->
However nothing is perfect and when the screen is resized to match the centre content width the 1px jog now appears on the left side of the layout!
This is unlikely to be a major problem but if it is then the only solution is not to use the body as a container for your background image but to use a 100% high container instead as in this example.
Back to original 1px Jog explanation
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