you could get the mouse position, but this is not the same as the one
you get on the image click  (imgbtn.x and imgbtn.y)
one is relative to the image and the other relative to the page

If that works for you then...
in your ImageButton you can try hook to the onclick event and
alert(window.event.x)

[c#]
imgButton.Attributes["onclick"] = "alert(window.event.x)";