I found this graffiti portrait of a girl at the Haludovo Palace Hotel  - a lost place on the island of Krk (Croatia) - and turned it into a HueForge.

The picture is 100mm X 150mm which fits perfectly into European photo frames.

Printing

Layer height: 0.04mm

First layer height: 0.20mm

Infill: 0%

Bottom solid layers: 999

Top solid layers: 0

Time saving: Add a height range modifier for 0 - 0.60mm with layer height 0.2mm to save a lot of time!

Colors: Besides black and white there's red for the lips and two layers of blue for tinting the otherwise gray picture. See details below. 

You can probably replace the tinting color with anything that's not too bright. For example I was playing with green and magenta in HueForge which both work as a replacement for blue. Of course you can also just leave out the tinting completely and start with white at 0.64, which will result in true grayscale.

Filaments Used:

PLA DasFilament Schwarz (Black) Transmission Distance: 0.5
PLA DasFilament Königsblau (Blue) Transmission Distance: 3.6
PLA Elegoo White Transmission Distance: 6.3
PLA DasFilament Weiß (White) Transmission Distance: 2
PLA DasFilament Schwarz (Black) Transmission Distance: 0.5
PLA DasFilament Feuerrot (Red) Transmission Distance: 7.4
PLA Elegoo White Transmission Distance: 6.3

Swap Instructions:

Start with Black
At layer #12 (0.64mm) swap to Blue
At layer #14 (0.72mm) swap to White (TD 6.3)
At layer #28 (1.28mm) swap to White (TD 2.0)
At layer #33 (1.48mm) swap to Black
At layer #35 (1.56mm) swap to Red
At layer #42 (1.84mm) swap to White (TD 6.3) for the rest.