Javascript Image Placement

So I am building a puzzle pipe game and wanted each level to have a different background. The background is loaded from an img src only problem the image at he moment doest move with the screen size... I always want it to be centred so you can see the middle of the image.

For example a background that looks perfect on a 13" screen on a 27" looks a total mess... Is there a simple way I can keep the image within the same place? I know I could set up a css rule however was wondering if there was way a way to add it into the js script?

So far my js script looks like this (for one level):

level1: {
                level: [[["0","1","0","1"],["0","0","0","0"],["0","0","0","0"],["0","0","0","0"],["0","0","0","0"],["0","0","0","0"]],[["0","0","1","1"],["0","1","1","0"],["0","0","0","0"],["0","0","0","0"],["0","0","0","0"],["0","0","0","0"]],[["0","0","0","0"],["0","0","1","1"],["1","1","0","0"],["0","0","0","0"],["0","0","0","0"],["0","0","0","0"]],[["0","0","0","0"],["0","0","0","0"],["0","0","1","1"],["1","0","1","0"],["1","0","1","0"],["1","1","0","0"]]],
                backgroundURL : "http://imagehere.co.uk".style.backgroundPosition = "center bottom"; 
            },

Of course that didn't work... but im guessing it would be something along those lines.

Thank you