var bg= "http://www.abload.de/img/background1920x1080l1zv4.jpg";

if(screen.width < 1921) bg="http://www.abload.de/img/background1920x1080l1zv4.jpg";
else if(screen.width < 801) bg="http://www.abload.de/img/background800x60069lxk.jpg";
else if(screen.width < 1025) bg="http://www.abload.de/img/background1024x768qjxmv.jpg";
else if(screen.width < 1153) bg="http://www.abload.de/img/background1152x864lzlom.jpg";
else if(screen.width < 1281) bg="http://www.abload.de/img/background1280x1024tnykc.jpg";
else if(screen.width < 1367) bg="http://www.abload.de/img/background1366x76859b6n.jpg";
else if(screen.width < 1441) bg="http://www.abload.de/img/background1440x900pnlpb.jpg";
else if(screen.width < 1601) bg="http://www.abload.de/img/background1600x9006qz6b.jpg";
else if(screen.width < 1681) bg="http://www.abload.de/img/background1680x1050dua0q.jpg";

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