While interactive picking is generally discouraged because of its non-reproducibility, occasionally it might be useful. Using interact= option with xtpen outputs mouse-click coordinates in a text file. However, they are Vplot coordinates, not easily related to physical coordinates of the image. Joe Dellinger has a more comprehensive plan for adding interactivity to Vplot graphics.
sfipick is a simple Tkinter script which allows for interactive picking. The interface is straightforward. Use left-button mouse clicks to add picks, right-button mouse clicks to remove wrong picks, middle-button to drag picks. The picks are written in a plain text file and can be processed later.