Clear defaults is not highlighted. When I used the camera for the first time it asked which picture app to use as default and I opted for Google picture app.
I want to change it to the gallery app.
Go to the Google "Photos" app in your app drawer -> long press -> Get details (top right options menu, although I'm using Smart Launcher so this could be different!) and then click "Clear Defaults" - next time you try to open a photo from the camera it should give you the option to choose the gallery app as the default!
Hope this helps...
btw, this phone is amazing - even the battery life, which I wasn't expecting to be fantastic, is pretty impressive - fairly constant use yesterday and was down to only 60% last night!