In the Graphics menu you can call the Drawing Tools palette. The last of the Tools section is a crosshairs icon (the tooltip says "Get coordinates"). Select it, and click on your image. The coordinates of the points should appear. You can copy the coordinates to the clipboard and then paste them into a notebook. You can click on multiple points and have all the coordinates in a list.
More info in the Documentation Center at howto/GetCoordinatesForPointsInAPlot
