in the level with lots of clocks (think it's 14) you hold onto/drag that sticker so it accompanies you into the final level. and after the credits you'll see a slot for that sticker!
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!
