Thanks, but it doesn't seem to be the case here (Win 10, 1703). When I choose "Open With" and then "Choose Program", it brings up the default application menu for the .PNG file type (attached image). .PNG is already associated with my preferred image viewer app, so there's nothing for me to do here. When you double-click an image inside of a note, it continues to open in the internal Evernote image viewer.
The interesting part is that if you right-click an image file inside of a note, the default option (indicated by bold text) is "Open". (Attached image.) If you click "Open", it opens properly in the external app.
So it appears that when you double-click an image inside of a note, it does not perform the default action. It is bypassing these open options and doing its own thing by opening in the internal Evernote image viewer.