Hi guys! I think there are some hidden pictures in the photos app, because when you don't set your favorite pictures it selects some which are actually not present in the collection and puts them on the live tile.
I don't know about the OPs concern but to update them manually go to settings>applications>photos+camera and tap on the "Shuffle Now" button to update the tiles.I have seen this when you delete a picture it could take from hours to days to remove it from slideshow