Hi Madison!
This is called the Artmap Perk. It was one of the perks obtainable from the Halloween Bundle, which is sadly no longer available.
However, pixel art on a map can also be made by claiming a 8 x 8 (64 chunk) square, which is the space of a map, and flattening it and making a pixel art on that map, which if you make an empty map and click it whilst on that land, it'll print the picture on the map!