STYLETHREAD -- LET'S TALK SHOP!

Does anyone know what it's called when you take a ton of pictures and make one big picture of something using like colored pictures to make the bigger picture? Like if you take 1,000 pictures and make a big picture of the world and only when you look up close you can see that it's made of tons of tiny pictures, but when you look at it far away it just looks pixelated? It's often used in advertising. Thanks!!