Re-setting a css style when a mobile phone user switches from vertical to horizontal view

I have a webpage with a background image that looks fine when I hold my phone horizontally. Here's the code for the background image:

body {background:url(images/CoupleInSilhouette.jpg) no-repeat;
background-attachment:fixed; 
background-position:center; margin: 0 auto;
background-size: cover
}

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But since the background-position is set to center, when I view the page holding the phone vertically, the left part of the image is hidden from view. Is there code that will move the position of the background to the right by a specified number of pixels? here's the page in question: www.dijitalrealm.com/JCMusic/ManOfMyWords.html

Thanks,
John