function previewImage(ctlName, newSource, smallWidth, largeWidth, blockIndex)
{
    var targetID = newSource.replace("_T" + smallWidth, "_T" + largeWidth) ;
    var ctl = document.getElementById(ctlName) ; 
    var divChildren = ctl.getElementsByTagName("DIV") ;
    for (var ii=0 ; ii<divChildren.length ; ++ii)
    {
        if (!IsPhotoDiv(divChildren[ii]))
            continue ; 

        if (!divChildren[ii].style.display || divChildren[ii].style.display == "block")
            divChildren[ii].style.display = "none" ;
        
        if (divChildren[ii].id == targetID)
        {
            divChildren[ii].style.display = "block" ;
            ResetImageTag(divChildren[ii]) ;           
        }
    }
}