The 3D engrave option is available for raster images. It should be available for draftboard, acrylics, and hardwoods. If you don’t have the option, it’s possible your file is a vector image.
As for the 3D engraving stuff, I think there was a good tutorial under tips and tricks. You have to create a file with different shades, but I’m absolutely terrible at it. I only got it to work when using the tutorial about using Meshlabs with a 3D file.