1. If the images are inside that 'Extra' section, they are specifically not in-game. The images you've mentioned are thrown in there for fun as some have asked for Amelia to eat others, so the Gallery is the closest the game will get to that.
2. Essence is currently for bragging rights, it doesn't accomplish anything for now. Eventually, the Essence will be needed for the final boss. Since the final boss isn't anywhere near being added to the game yet, it's just for fun for now. Some Essence that is hard now should become easier over time as Vigor and Technique are very important for it, and as new predators get added, Amelia's stats can increase.