The short answer
Choose a light, even-grained wood, prepare the photo for a narrow tonal range, and let the bare wood be your white. On wood the highlights are not engraved at all — they are the untouched surface — so anything that lightly dots those areas immediately reads as a grey haze over the picture. Then run a test grid for that species, because a setting that suits birch will not suit walnut.
Light, even grain beats figured wood for photos
Highlights = bare wood, so they must stay unburned
Test per species, not once for 'wood'
What the species does to your photo
Light woods with little grain contrast give you the widest usable range, which is why birch plywood is the default test material in most workshops. Darker woods such as walnut start with less contrast between burned and unburned areas, so the image needs more separation in preparation to stay readable. Strongly figured woods add their own pattern on top of yours — beautiful for signage, difficult for faces. Resin pockets and knots burn differently from the surrounding area and will show as blotches.
Birch and maple: widest tonal range
Walnut and dark woods: less contrast, needs more separation
Figured grain competes with the subject
Why wood engravings turn out muddy
Three causes, in order of frequency. The light areas are not really white, so the whole background picks up a faint dot pattern and the picture loses its sparkle. The mid tones are compressed too far, so the face becomes one flat area. Or the dot pattern is too fine for the burn spread, so neighbouring dots merge into a solid tone. All three are preparation problems, not machine problems — which is good news, because they cost nothing to fix.
Near-white that still burns creates a grey haze (measured: 2.6% dots at brightness 248)
Over-compressed mid tones flatten faces
Too fine a dot pattern merges into solid burn
How Vextrify helps
Photo Studio carries a profile per surface — light wood, dark wood and the other common materials — so the tonal work starts from that material rather than from a generic filter. White areas in your image stay untouched material instead of collecting a light dot pattern. When the image is ready, the same project goes to the machine: place it on the real workpiece with the camera, trace the outline with a dry run to check the position, then run it. The browser version is free and needs no account.
Per-material profiles for light and dark wood
White stays bare wood
Camera placement and dry run before you burn