Verizon does not have a very good instruction manual for this phone.
That is because Verizon does not make the phone and therefore does not make the manual for the phone.
You weren't asked to choose an app to look at the photos when you took them because you were already using an app, the camera, and didn't need to open an app in order to look at the photos.
Normally when you look at photos, you will use "Gallery" or "Photos" however there are other apps you can install in order to look at them. If you are already in an app which has the capability to open photos, you would not be prompted to choose another app to open in order to view them.
Since you are simply trying to look at your photos and not simply reviewing them after taking them, you don't already have an app open with photo viewing capability. Your phone, therefore, is asking you which app you would like to view the photos with. Try both, use the one you prefer.