I've started gathering photographs of HAs, that are currently missing in the game (atleast in V 1.26, if there has been a major update for V 2.0, this might be for naught). If possible, I'd like to replicate the slightly brownish and grainy look of the stock photos, so they blend in better. Any pointers on how I might achieve this? I fiddled around with GIMP, but so far without satisfactory results.

In Photoshop you first select the "Mode" for the image as RGB 8bit,
if it was black&white before.
Then you have under "Corrections" the choices "Variations" and/or "Colour balance".
With both you can easily bring a red-brown, or yellowish tone into the image.