2 Of them are unlocked by typing in "canonending" when the game asks you to input your name. A bunch of them are shown based on a choice you make if you get all 50 choices correct. If you get all 50 choices correct, Yuka will tell you that you can get 1 reward from her and you can choose which "reward" you want.
It should also be possible to go to the image subfolder in the "game" folder inside the .zip file to see all the images
Ps. thanks for your support :D