How to associate alternative app to stock Gallery app when launched from stock Camera app
the answer is you can't ,because the "preview" button action is hard coded in the stock camera app as a link to the right activity in the stock gallery app it sends an intent with the path to the jpeg image to the stock gallery ,the link is not an intent that can be received  by third party apps ,even if you try deleting the stock gallery app the preview button will have no action and in some phones your camera app will force close.