When I use teh preview pane I wast dOpus to show the pictures.
But when double-clicking them I want to open them in a default application as I configured in Windows.
99.9% of teh time I use the Commander layout.
I have my images set to do this, only I use MaxView as my default viewer. If I double-click an image it opens in MaxView, if I select an image and press the "Show" toolbar button then it opens in the Opus stand alone viewer. And of course the preview pane works independently as yours does.
If you edit the Recognized Images file type (Settings/File Types) to be similar to the screen grab of mine below, I believe it should work as you want.