You may overlay a coordinate graticule over the displayed viewer maps using the Graticule Settings dialog.
The graticule spacing is defined using the ground units of the specified coordinate system. Checking the Link Spacing checkbox will keep the Easting and Northing values synchronized as either one changes.
By default, the Source Image Viewer and Reference Image Viewer assume the Reference Coordinate System for the graticule. If you use a different graticule coordinate system from the viewer, you can also specify the datum transformation between the two systems for the best results.
Clicking the Graticule Appearance… button will allow you to change the colors and fonts used with the graticule.