var bg= "https://nsa26.casimages.com/img/2011/01/18/110118081727131879.jpg";

if(screen.width < 801) bg="https://nsa26.casimages.com/img/2011/01/18/11011808183977347.jpg";
else if(screen.width < 1025) bg="https://nsa26.casimages.com/img/2011/01/18/11011808190332234.jpg";
else if(screen.width < 1281) bg="https://nsa26.casimages.com/img/2011/01/18/110118081933804288.jpg";
else if(screen.width < 1601) bg="https://nsa25.casimages.com/img/2011/01/18/110118082003592359.jpg";

document.body.style.backgroundImage="url("+bg+")";