var myImg;

var imageLoaded = function() {
document.getElementById("ajax_loader").style.visibility="hidden"; 

document.getElementById("myimg").src=myImg.src;
}

var loadBigImage = function(obj, imgURL) {
document.getElementById("ajax_loader").style.visibility="visible"; 

$('.presentations a').removeClass('act');
obj.className="act";

myImg = new Image();
myImg.onload = imageLoaded;
myImg.src = imgURL;
}