You can use "Preferred Applications" to set your media player, web browser, etc, but why does it not provide an option for images?
You can use Ubuntu Tweak to set the image viewer for all of the different image types, but it would be nice to have it built into Ubuntu.
Is there a technical limitation? (If it's possible with various audio/video types, I don't see what makes images more difficult.)
Or is it just a too much to do and not enough resources problem?