Makerspace laser cutters also work well as engravers, and with them you can engrave on wood, acrylic, metal, and more. High-contrast images usually work best for engraving - here are some tips for optimizing your photos.
Step 1: Find Some Art You Like
See this Instructable for how to download public domain art from museum websites: https://www.instructables.com/id/Public-Domain-Ima...
Art that will work well for laser engraving will most likely have just a few colors or tones, and should probably be bold and simple. A lot of printed art - woodcuts, engravings, or lithographs - will work well because the process used to make the original art has limited colors or bold lines.
There are plenty of public domain images from the masters of the Japanese arts of ukiyo-e and shin hanga.
Here's some public domain shin hanga: http://collections.artsmia.org/search/%22shin%20h...
And engravings and woodcuts are usually easy to find on museum websites.
Here's Beham’s Hercules capturing Cerberus, from 1545: http://collections.artsmia.org/art/124317/hercule...
Step 2: Grayscale Your Image
A makerspace laser cutter can’t engrave all the tonal gradations (or colors!) you see in a print, of course, so you’ll have to reduce the tones in your image.
Open your image in your favorite image editing tool. Change it to black and white.
Step 3: Add Contrast and Separation
Make sure you’re using all of the possible tones in the image - set the white tones to white, and the black tones to black. You don't want your lightest tones to be less than white, and you don't want your blacks to be lighter than black.
(You don't have to use contrasty images, but it will be easier to be consistent among engraved photos if you're always using pure black and pure white as the end points of your image.)
If there are tones you know you want to keep, separate them using Curves or Levels adjustments.
Save a low-resolution version of your image - laser engraving is not very high-resolution. Images less than 2000 pixels on their long side engrave just fine, even at 8x10 or 11x14 inches.
Step 4: Test Power and Speed on Your Laser
Try both different speeds and different power settings on your laser engraver. These are test images for an 80W laser engraver, and I've put the text into the image so I can tell the images apart when they're done.
Step 5: Decide on Your Favorite Power / Speed Combination
Choose the combination that's best for your image.
Step 6: Make Yourself Some Coasters!
The wooden coaster in the second photo has been treated with shellac, which seals the wood and makes the engraved areas much stronger.