I had to face this issue quiet a few times but couldn’t find any solution. I’ve searched for it thousand times may be!!
Whenever I add an image background to a full width div and I check it in the browser a thin white line of extra background appears as well as the scroll bar does. The document takes some sort of margin I guess.
Is it a background-image or are you using an image as background? We can’t debug a screenshot, but in case of the last mentioned approach you could try setting the image to display: block.