Ah I get your frustration. Have you thought about looking for Open Domain art that might work? It's what I did for The Thirteenth Damned Son. I can point you towards some things.
I wish I could help with suggestions, besides use heavy Ink lines, good lighting, and no flash; and up the contrast when you have it on your image editor, it should make it easier to take and edit in a program like Gimp.
Viewing post in How do you all cope with 'Freezing creatively'
I am a master at Gimp and went to collage for graphic design. I just drew a version of Tuesdays Demon that im pretty happy with after about 3 hours of work. I can only really draw humans at lever 5 skill, and alot of folklore creatures are animals with wild biology. I never really use ink in my drawings (Tuesdays Demon is done is Artist's Pencils) unless im drawing in a cartoon style, as im much better at shading that at cross hatching (even if i think cross hatching often looks cooler). Ill try to snap a good picture with no flash and a bunch of soft lights, thanks for that. Im sure I can adjust the levels in Gimp to get it natural looking. Thanks.
anyone reading later feel free to look at my old art here https://imgur.com/user/NellTullDisturbingArt