I hadn't searched their individual sites until just now when I used the links you very graciously provided! Unfortunately no positive results.
However, since first starting this thread a couple of days ago, I did hear back re: an image at the Royal Collection Trust, whose written description fits what I am looking for. Unfortunately they have not yet photographed the c.1880 photo in question so as to turn it into a digital image, which means I will have to pay a rather stiff fee for them to do so and then send me a digital copy -- at least I think that's what they would do, as I doubt the price, high as it is, would cover a hard-copy of the image, but maybe it would.
I still haven't decided whether or not to spend the money needed to get a copy of the image, though at this point I am unfortunately somewhat convinced that is probably my only chance to get such an image, as I really doubt any has escaped my intense and ongoing online searches... but you never know for sure. I may send individual email inquiries to NAM and the Anne SK Brown collection to see if they can perhaps dig something up in their own archives which is not yet available on their websites.
Again, thanks very much for taking the time and making the effort to help me out, it is very much appreciated!
PS Funny, I recently visited the Anne SK Brown collection at the John Hay Library at Brown University in Providence, Rhode Island, for the first time ever, and was somewhat awestruck by their vast collection of toy soldiers, which includes a large number of Victorian British and Indian pieces.