I have just migrated from Win 7 to Win 10, have removed all Win 10 photo apps and have activated the Win 7 photo viewer.
This all works fine in the native Windows explorer (see explorer screenshot), but in Opus when i right-click on an image i see the Win 10 photo gallery viewer listed (it doesn't work) instead of the windows photo viewer (see opus screenshot).
Obviously, I don't know what the original poster did, but, for information, Winaero Tweaker provides an Activate Windows Photo Viewer function which is what I used.