cropped version of geeketteSpeaks-related solution:
<html>
<head>
<script type="text/javascript">
function getImageCoords(event,img) {
var posX = event.offsetX?(event.offsetX):event.pageX-img.offsetLeft;
var posY = event.offsetY?(event.offsetY):event.pageY-img.offsetTop;
alert("You clicked at: ("+posX+","+posY+")");
}
</script>
</head>
<body>
<img src="image.jpg" onclick="getImageCoords(event,this);">
</body>
</html>