How to open Image files in other applications (not in the Opus viewer)?

I need to open image files (e.g. .PNG) by double-clicking in Opus in a custom application. But when I double-click a .PNG file in Opus, it always opens in the Opus viewer. So I have tried to set this option in Opus Preferences:

image

But now the double-clicked .PNG file opens in the Opus HEX VIEWER!

Of course, I have tried to use the Shell Open With menu to (re-)assign .PNG files with my custom application. But it does not work: It always opens in the Opus Viewer.

Of course, it does work in Windows File Explorer. But how can I automatically open my custom application when double-clicking an image file in Opus?

Preferences / File Operations / Double-click on Files / Use internal picture viewer for...

Thank you! Why we often don't see the most obvious things?

There's a FAQ as well: