The next update will considerably increase the chance of getting the images you haven't seen yet to unlock the rewards, and also introduce new ways of seeing some of them with a new game mode. The update will be out this week.
In future updates, there will probably be more game modes and also difficulty levels. This will, hopefully, increase replayability and provide more ways to organically unlock all the rewards, without making it feel grindy and tedious as is right now.