In order to get a full heart, you must find 3 heart fragments. You can get one in each Chapter so by Chapter 3 you will get this achievement.
Chapter 1
Before entering the school, go to the top-right exit of the grounds to find a circle of mushrooms to the left, which is the puzzle hint for the heart piece puzzle on the other side. Take the picture and remember the pink colors. Go to the right and cross the trees to see the puzzle in which you get the heart piece.
Chapter 2
After you receive the Hockey Stick in the basement and continue through, you will reach the left side path and there will be an equation with markers on a chalkboard. Take a picture as it is the solution for the puzzle. Below, there will be a room with another heart piece puzzle.
Chapter 3
After you leave the campsite, go to collect the red skulls to activate the stones blocking the road. Go to the bottom of the screen where you will see a lake. The solution to the puzzle is in the lake so take a photo. As you head back you can go between the tress for the puzzle with the hear piece.
Once you get it will unlock this achievement.