i cant seem to upload a screenshot... but it's the eight-pointed star sticker (star/sun-shaped) in the last screenshot, right next to the woman in dark dress. once you came across it, you just drag it from one mini-game to another. in the final level you will see a slot of the same shape!
ok i think i misunderstood, so if you are asking how to get pass the level as in the second screenshot... you got the body anatomy sticker right? then you drag the fire sticker onto it, and the two stickers turn into a single key. then when the man in the background appears, you drag the key onto him, and you will get a different key with two holes in it. click on both holes.