This Liquib scene shows the Mosaic Tool applied to a photo of M & M candy within a dispenser.  Tile Shape was set to Triangular, Tile Content to Averaged Colors, Tile Size to 22 and Shape Regularity % to 22%.  The fairly low Tile Size and Shape Regularity % values allow the rendering process flexiblity with adjusting the tile shapes to match the source image elements.  If Shape Regularity % had been set to 100%, the tiles would be shaped into perfect equilateral triangles, but they wouldn't represent the source picture nearly as accurately (unless perhaps the Tile Size was set quite low).