The Glowforge tutorials on editing with Gimp or Photoshop cover what you should be looking for, generally high contrast as I said earlier. Also, forget 3D engrave / vary power for photographic engraving.
The only way to know is to try a small section of your images, on your materials, with various settings. I have a small stack of test samples like that. Just don’t expect the “HD” defaults to be better than “Draft” - but you’ll quickly find out if you try them yourself.