do you need to postback the page? or you can use a client side solution?

Is there a client-side solution which
will get me the x,y coords of the
mouse position? If there is it would be
better.

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)";

I've never knew about this!
I can do some arithmetic and
get the same coords as the
ImageButton and I won't even
need to use the ImageButton,
I'll just use Image and do it all
client-side.