View in Opus but keep icons

How can I get Opus to be the default handler for all images and keep the default Windows 8 icons for images?

You would probably have to edit the registry, since the standard Windows Open With method of assigning the Opus viewer to the filetypes will also change the icons.

(Assuming we are talking about using the viewer outside of Opus. If you only care about what happens inside of Opus then you can leave the Windows 8 viewer associated with the filetypes and just turn on the option in Opus to make it use its own viewer whether or not it is the system default.)

The question that is asked when you install Opus; Do you want to make Directory Opus the default handler of images. How do I get back to that choice? I think that once I installed it and I got just what I wanted; The Windows icons but with Opus always opening the file to view in external opus viewer. Is there any way in that viewer to launch the file in another application?

That choice only affects what happens inside of Opus, and won't affect the icons either way.

See these two FAQs for more:
Stop the Opus image viewer from being used by default
Use the Opus image viewer inside or outside of Opus