var bg= "https://i1097.photobucket.com/albums/g349/perezperez22/bg4_zpse8d80076.png"; 
 
if(screen.width < 801) bg="https://i1097.photobucket.com/albums/g349/perezperez22/bg800600_zpsf59485c9.png"; 
else if(screen.width < 1025) bg="https://i1097.photobucket.com/albums/g349/perezperez22/bg1024768_zps9cc561db.png"; 
else if(screen.width < 1281) bg="https://i1097.photobucket.com/albums/g349/perezperez22/bg1280960_zpsf81a24da.png"; 
else if(screen.width < 1601) bg="https://i1097.photobucket.com/albums/g349/perezperez22/bg16001200_zps34ae73e3.png"; 
 
document.body.style.backgroundImage="url("+bg+")";