The discover feed creates pre-generated images from your camera roll. The system does its best to analyze your images and give the best results. Over time it will improve and give you even more verses relevant to your pictures.
The app pulls from your camera roll three photos every day that you open the app (if there are three available for use in the app), and if you would like to see more photos in your discover feed you can pull down on the feed to refresh the list, and three more images will load.
You can also upload to take a photo anytime by clicking on the "camera" icon.