This portrait is pixelated in Photosphop. The keyword here is: "Nearest Neighbor" (Preferences > General > Image Interpolation: Nearest Neighbor (preserver hard edges)).
It took me some time to figure that out. Make a pixelgrid by using guide layout. Then pixelate your source image, get a squared brush and you're good to go. Play around with colors and shapes. Try to use just around 16 different colors. Don't worry too much about mistakes. It takes some time.
But once you are finished, it's so much fun, easy and rewarding to create many different looks like monochrome, 8-bit (4-8 colors) and so on with the GIF export settings. So remember for philosophy: Just your nearest neighbor can preserve hard edges ;)