You might have a corrupt photo causing the app to crash. The first thing that I would try is to completely close the photos app and restart your phone. Slowly swipe up from the bottom of the screen to get to the app switcher. Swipe to find the photos app preview window. Swipe up and off the top of the screen on that window to close the photos app. Then restart your phone and try again.
Restart the phone by holding the sleep switch on the right while holding down on the volume up button on the left. Then swipe the slider at the top of the screen to shut down. You can also shut down in Settings>General>Shut down.
After the phone shuts off, hold the sleep/power switch on the right in order to restart.