The Pixel fractal formula is a simple stub formula that marks all pixels in the layer as outside. This gives exclusive control over the layer to the coloring algorithm selected on the Outside tab of the Layer Properties tool window.
Combined with a direct coloring algorithm such as the standard Image coloring algorithm, this makes it possible to import images, as shown here. See Using images for more information.
Pixel is available as a fractal formula in Standard.ufm and as a fractal formula plug-in in Standard.ulb.
Image coloring algorithm