The 1x1-* images are all exactly one pixel.

The swatch-* images all use 15x15 cells.

The square-* images all use 15x15 cells with one pixel borders.

The pattern-* images use cells of various sizes:

   pattern-gg-gr.png         12x12
   pattern-grg-rgr-grg.png   14x14
   pattern-rgr-grg-rgr.png   14x14
   pattern-tr.png            15x15
   pattern-grg-rrg-rgg.png   15x15
