Determine the top corner of the component you use to display the image and use those as an offset.
For example if the image is displayed from 100,100 on your application window and your mouse coordinates are 150,150 your coordinates in the image would be 50,50 (or was that 49,49 ...).

Mind this is just an example, I haven't checked if the coordinates in either case are calculated from the top left corner of the application window (some libraries have a tendency to calculate from the bottom right corner in which case you need the window size as well to calculate another offset to determine the top left corner).