var imgnum=0;

function change_style(){

switch(imgnum){
case 0:document.getElementById("bg_header").style.backgroundImage="url(../../img/bg_header/bg_header01.jpg)";break;
case 1:document.getElementById("bg_header").style.backgroundImage="url(../../img/bg_header/bg_header02.jpg)";break;
case 2:document.getElementById("bg_header").style.backgroundImage="url(../../img/bg_header/bg_header03.jpg)";break;
case 3:document.getElementById("bg_header").style.backgroundImage="url(../../img/bg_header/bg_header04.jpg)";break;
}

if ( ++imgnum > 3 ) imgnum = 0; 

setTimeout("change_style()",10000);
}