There are several options, paid and free. Search for 'Photoshop Dither Plugin' and you will find them.
If you are planning to do a lot of photo engraving I would suggest to purchase Photograv, it is much easier and saves wasted material.
PhotoGraV - The Power Tool for Laser Engraving Photographs
The same results are possible with Photoshop but you have to spend more time on the setup and aren't making money.