var myPix = new Array();
var imgCt = 6;

function PreloadImages(){        
    for ( var i = 0; i <= imgCt; i++ ){
        myPix[i] = new Image(400,266);
        myPix[i].src = imagePath + i + ".jpg";
    }
}
function ChoosePic() {
    // Assign the image src property via the random number
    var randomNum = 1;
    randomNum = Math.floor((Math.random() * imgCt));
    document.getElementById(ctrlPrefix + "_imgMainPhoto").src = myPix[randomNum].src;
    //alert("Image #" + randomNum + " is set.");
}