Recommended method is here: How to use the Opus image viewer inside or outside of Opus
The problem with the more general "default photo viewer" setting in Windows is there are lots of types of photos/images, which not all viewers show, and the Opus viewer can also handle lots of file types which you may not all want to be sent to it. We feel it is better to set things up for each type/extension instead of all at once. (You're unlikely to want Word documents or MP3 files opening in our viewer on double-click, for example, even though it can handle them.)