var bg= "Die Adresse des Hintergrundbildes mit der Höchsten Auflösung (größer als 1600 Pxiel)";

if(screen.width < 801) bg="https://i.servimg.com/u/f28/16/28/29/16/back8010.jpg";
else if(screen.width < 1025) bg="https://i.servimg.com/u/f28/16/28/29/16/back1010.jpg";
else if(screen.width < 1281) bg="https://i.servimg.com/u/f28/16/28/29/16/back1210.jpg";
else if(screen.width < 1441) bg="https://i.servimg.com/u/f28/16/28/29/16/back1410.jpg";
else if(screen.width < 1601) bg="https://i.servimg.com/u/f28/16/28/29/16/back1610.jpg";
else if(screen.width < 1921) bg="https://i.servimg.com/u/f28/16/28/29/16/back1910.jpg";

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