After tapping Choose Shots, first tap the edit button at the bottom of the screen below the photos on your Camera Roll.
Then find the shot for which you want to turn off movement, and tap that photo.
In the photo editing window, you can spread your fingers apart to zoom in on a part of the photo or pinch your fingers together to zoom out.
You can also reposition the photo by dragging within the window with one finger.
Then tap Save to save the change you've made to your photo.