Hello Doctor Dink !
Sorry for my English I just wanted to warmly thank you for your curiosity and your kind words !! We translated the game in english especially so that english speaking people could experience it so your post make me really happy !
once you find the 5 grey images in the credits, you have to find to which page of the site they refer to (based on the little shape you see, a moon, an eye... and the red dot shows you where you have to click once you find the corresponding page... and then it leads you to secret content :) If you find the 5 hidden red spots and see what it hides you have finished the game ! Hope that helps ;) !