Make a folder with at least two photos, enter the folder, select any photo to view it, and simply tape down the d-pad or rubber band the left stick to the left or right to quickly scroll through all the photos in the folder non-stop.
This method takes about 10 hours to complete, so it is best to leave it on overnight while you sleep. Once the requirement is met, you must then exit the folder for the achievement to actually pop.
To view the precise number of photos you have viewed, go to the dashboard, highlight the app and press the menu button on your controller. Select "View app details" from the list and then scroll to the right to view the app leaderboard.