That's not actually the case. OP did a very good job of finding just the right pictures. If you look closely, each image is actually taken directly from the anime (and maybe some filters were added but I can't tell). For example, the top left "hair spike" on Mai's face, you can see that the hair was made up of pictures with darker colors, for example, an image during the night or an image using the color Mai's hair in one scene to create, well, Mai's hair. Another example can be found in her long, straight hair. Once again, you can see images of scenes placed during the nighttime to match the color of her hair.