var bg= "https://i.servimg.com/u/f23/16/43/58/42/bg110.jpg";

if(screen.width < 801) bg="https://i.servimg.com/u/f23/16/43/58/42/bg800_10.png";
else if(screen.width < 1025) bg="https://i.servimg.com/u/f23/16/43/58/42/bg102410.png";
else if(screen.width < 1281 && screen.height < 961) bg="https://i.servimg.com/u/f23/16/43/58/42/bg128010.png";
else if(screen.width < 1281) bg="https://i.servimg.com/u/f23/16/43/58/42/bg128011.jpg";
else if(screen.width < 1441) bg="https://i.servimg.com/u/f23/16/43/58/42/bg144010.jpg";
else if(screen.width < 1601) bg="https://i.servimg.com/u/f23/16/43/58/42/bg160010.jpg";
else if(screen.width < 1921) bg="https://i.servimg.com/u/f23/16/43/58/42/bg110.jpg";

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