Pure black and white (not grayscale) source images produce excellent burnt images. They look razor sharp close-up and far away.
High contrastgrayscale and colour images provide much better results than low contrast images.
Avoid borders and frames around the image - they tend to be distracting, and not in a good way.
Dithered engraved sections generally enhance the visibility of the wood's grain.
Note: Remember that close-up, grayscale and colour images can sometimes look "dotty" from the dithering. When viewed from at least 50cm away, your eyes will merge the dots into shades.