The picture on the sticker is printed, just like in a book. The original picture or photograph is printed by a machine called a printing press.
What is mysterious is the sticker part on the back of the sticker. When you peel off the plastic-like paper, it comes off easily, but once you put it on a wall or a pen case, it sticks to the wall or pen case perfectly and does not come off easily. This is because the plastic-like paper on the back of the sticker is coated with a special release agent.
The release agent is made of a material that does not get along well with the adhesive. So, even if the sticker is attached, it will peel right off if you try to peel it off.
However, if you remove the sticker from the plastic paper and place it on another surface, there is no more release agent on the surface, so it will adhere perfectly and will not come off.