Images are one of the most difficult things to do …
I suggest you watch the Laser Everything video on photo engraving. I recommend this to anyone as it explains how to select the proper interval for anything you wish to do.
You have to understand what the preview is actually is showing you. It has no knowledge of what material, laser or anything else about your project… it only shows you where the laser will lase.
Generally speaking, you want to use a dither. This is similar to how a printing press works as far as being only able to make a burnt spot or nothing. 256 shades of gray are difficult to obtain with any material… stick with a dither of some type. Stucki is used quite often.
Here’s a thread that may help. You can use the search function to find other information on this site for types of ways to make photographs or something similar…
Good luck
