/*
 * preloading an image. creating an image object with given source
 * imgObj: name of object to be created
 * imgSrc: URL of image 
 */
function preload(imgObj,imgSrc)
{
	if (document.images) {
		eval(imgObj+' = new Image()');
		eval(imgObj+'.src = "'+imgSrc+'"');
    }
}

/*
 * change a layer's image elemnt's source
 * layer: name of a non nesting layer. if null look for image in document.images
 * imgName: name of image element
 * imgObj: name of preloaded image object
 * -> image element of layer source would be replaced with image object's source
 */
function changeImage(imgName,imgObj,layer)
{
    if (document.images) {
        if (document.all || document.getElementById || layer == null)
            eval('document.images["'+imgName+'"].src = '+imgObj+'.src');
        else eval('document.'+layer+'.document.images["'+imgName+'"].src = '+imgObj+'.src');
    }
}