I tried engraving some tiles from Lowes. They worked better than expected and only cost about 15 cents each.
The left one was engraved at 40% power, 325 mm/sec speed and 0.065 mm scan gap.
The one on the right was engraved twice at 14% power yielding a light gray image each time, So I then wiped it with black acrylic paint for a dark image. I assume the slight loss in resolution is caused by engraving it twice.
ernie