In the Dopus viewer, there is no way to open the currently viewed file in an external programme.
You have to go back to the file manager and find the file - if you have been browsing in a large folder of images that can take a while as the focus is still on the image you opened the viewer with.
I can think of three posibilities:
1) add "copy filepath" to the viewer context menu 2) add "open with" (as in explorer menu) to the viewer context menu 3) add "open in external editor" to the viewer context menu, with ability to set the external editor in preferences.
I'd be happy if all three were added but the filepath option is the most flexible, and can be used to open and/or import the file in any other programme, be it screenshot programme, image editor, office software, etc