See Also: How to use the Opus image viewer inside or outside of Opus to do the opposite to this FAQ.
When you first install Opus, a wizard asks you a few questions, including if you would like Opus to handle double-clicks on image files. If you say yes to this (the default choice), Opus turns on the Preferences option Preferences / File Operations / Double-click on Files: Use internal picture viewer.
With this option on, double-clicks on recognized image files in Opus are automatically trapped and the internal Opus image viewer is used to display the image. This option does not affect what happens when you double-click on a picture in Explorer or any other program - it is simply an override to the double-click action within Opus itself.
If for whatever reason you don't like the Opus image viewer and wish to go back to the default Windows one, or Photoshop, or whatever you normally use to open images on double-click, simply turn off the option in Preferences mentioned above.