Open pictures in the image viewer by default
If you want to make the image viewer the default program that opens certain image file types:
In the files app, right click the file and choose Properties.
Go to the Open With tab. A list of Recommended Applications will appear.
Select the Image Viewer and click Set as default.
If Image Viewer is not on the Recommended Applications list, click Show other applications. Select Image Viewer from the list that appears and click Set as default.
In future, when you double-click a file of the same type, it will open up in the image viewer.