ASCII art. The picture is covered with a grid of text characters, each chosen so that how much ink it puts on the page matches how bright that part of the picture is: light punctuation for highlights, dense blocks for shadows.
The characters are drawn from small bitmaps rather than loaded from a font, so there is nothing to download and the shapes stay crisp at any size.
Each cell takes a single reading from the middle of its patch rather than averaging the whole patch. That is deliberate. Averaging gives you something closer to a photograph, while single sampling gives the harder, more posterised edges that read as a terminal. To stop flat areas turning into long runs of the same letter, each cell rolls a die and picks between three characters of equal weight.
In colour mode the character is tinted with the colour it replaced, so the text carries the picture's palette.