var bg= "http://www.abload.de/img/1600x12003z2s.jpg";

if(screen.width < 801) bg="http://www.abload.de/img/800x600sz85.jpg";
else if(screen.width < 1025) bg="http://www.abload.de/img/1024x768jaiz.jpg";
else if(screen.width < 1281) bg="http://www.abload.de/img/1280x960el5g.jpg";
else if(screen.width < 1441) bg="http://www.abload.de/img/1440x1080tlif.jpg";
else if(screen.width < 1601) bg="http://www.abload.de/img/1600x12003z2s.jpg";

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