can someone help me with the image elisp API

[Jim Newton]

I've opened an image (a .png) file in an emacs buffer.
I'd like to be able to click in the image with the RMB (or any good mouse 
event),
and somehow relate the point the user clicked to the 
coordinates in the image.

For example, this elisp function gets the x-y position.  Suppose it is (609 . 
41)
I want to know if that is in the box within the image coordinates
 (( 447.32 199.28) ( 449.7426 199.9871))

I need of course to consider that the user might have scrolled or panned or 
zoomed
within the image before calling the class-graph-from-click function.

(defun class-graph-from-click (event)
  (interactive "e")
  (let ((position (event-start event)))
    (posn-x-y position) ;; this is the x-y where the mouse was clicked-down
    (message "xy=%s" (posn-x-y position))))