[TheSarce]

Looking at that "artwork" again, it seems to me that this is alaso a tracing off a photo...

Yep. Make sure you check out the posts at #167, 168, and especially 187 and 189. Make sure you're not drinking coffee or you'll lose your keyboard! ;-)

You've got some art background, right? How do we track down the other originals? I've been searching through Google Images, but it's just hit and miss hoping some combination of keywords brings something up.

[Miss Marple]

I don't know. I have tried searching google under the following phrases:

Sioux
Cheyenne
Nez Perce (I happened to notice that Colorado has a Nez Perce collection in the department of ethnic studies)
Chief Joseph (because of the Nez Perce thing)
two Indians
19th century Indian photographs

So far I have found nothing.

I am going to look at the picture again to see if it gives any clues.