identify x,y coordinates in an image

[John Hunter]

I need to get the x,y coordinates of a bunch of points in a 2d image.

I would like to mouse click on a point in the image and get the
coordinates of that point with reference to some origin (also
identified with mouse click) and some x,y scale which could be
customized.

I will need to be able to zoom in at some points of the image, and
perhaps scroll because the image will be large.

I suspect some combination of PIL and Tk is what I an looking for, but
am soliciting advice and/or prototype code before I begin.

Thanks,
John Hunter