Use this script

var isOpera, isIE, isNav, isFox, isOther = false;
if (navigator.userAgent.indexOf("Opera")!=-1) {
isOpera = true;
} else if (navigator.userAgent.indexOf("Firefox")!=-1) {
isFox = true;
} else if (navigator.appName == "Microsoft Internet Explorer") {
isIE = true;
} else if (navigator.appName == "Netscape") {
isNav = true;
} else {
isOther = true;
}

if (isIE)
{
OffsetX = window.event.offsetX;
OffsetY = window.event.offsetY;
} else {
OffsetX = e.pageX - document.getElementById("divImage").offsetLeft;
OffsetY = e.pageY - document.getElementById("divImage").offsetTop;
}