3d lasers engrave point clouds into crystal. To engrave a photo it must be converted into black and white. My initial workflow was to create an 8 bit grey scale image in GIMP and then convert it into a 1 bit, black and white, bitmap using a package called 2d Photo, then into DibSlice or LaserImage to create a point cloud. I'm now able to skip 2d Photo by creating the black and white image in GIMP.
To do this, go to the dropdown menu Image > Mode > Indexed
Use black and white.
If you want to convert a photo you'll want to select Color Dithering and try Floyd-Steinberg (Normal). The other dithering options may also be useful, have a play. Once done save the file in your preferred file format, .bmp in my case.
GIMP is free to download and use.