function swapback()
{document.getElementById("back").style.background = 'url(assets/images/banner_2.jpg) top center repeat-x'; setTimeout(swapback2,3000);}
function swapback2()
{document.getElementById("back").style.background = 'url(assets/images/banner_3.jpg) top center repeat-x'; setTimeout(swapback3,3000);}
function swapback3()
{document.getElementById("back").style.background = 'url(assets/images/banner_1.jpg) top center repeat-x'; setTimeout(swapback,3000);}

function swapbacks()
{
setTimeout(swapback,3000);
}

swapbacks();