Well Michael, if your into graphic design, you should know that if you have a crappy photo (low res) your gonna get a crappy result. You photo needs to be 200-300 ppi and engraved at the same resolution. I do prep in Photoshop CC, save as greyscale and then use another program called Photograv 3.0. The Gold method is a set of actions for use in Photoshop and can be downloaded from this site. Just do a search for it and you will find it. Works pretty well.
Remember also that any natural material such as granite can vary widely in quality. If you can find absolute black marble it is probably a better choice than granite tiles that come from the big box stores.