How does one use the Open With External Editor feature of Shotwell?

Question

• Shotwell 0.20.2 > Edit > Preferences > External Editors

  External photo editor: [GIMP Image Editor]

• Clicked on images so that they have blue borders.

• Photos > Open With External Editor

but the Open With External Editor is greyed out. How does one use the Open With External Editor feature?

Edit photos with an external program

Answer 1

Though the title says Edit photos with an external program the description says:

select a photo

One at a time works but not multiple.