function preloader() 
{
     // counter
     var i = 0;

     // create object
     imageObj = new Image();

     // set image list
     images = new Array();
     images[0]=""

     // start preloading
     for(i=0; i<=20; i++) 
     {
          imageObj.src=images[i];
     }
}

var imgs = new Array(6); 
    imgs[0] = "./images/1_o.png"; 
    imgs[1] = "./images/2_o.png"; 
    imgs[2] = "./images/3_o.png"; 
    imgs[3] = "./images/4_o.png"; 
    imgs[4] = "./images/5_o.png"; 
    imgs[5] = "./images/6_o.png"; 

function preload() { 
    var tmp = null; 
for (var j = 0; j < imgs.length; j++) { 
    tmp = imgs[j]; 
    imgs[j] = new Image(); 
    imgs[j].src = tmp; 
} 
} 
void(preload()); 

function imgswap(img, swap) { 
    img.src = imgs[swap].src; 
}