I don't know the answer but if you find the images I'd be careful about deleting them. There's obviously something about the Fire that Moon+ isn't taking into account and if it doesn't find those images who knows what might happen.
If you find them an alternate solution might be to replace the one that it keeps reverting to with another image with a color of your choosing.
I had a couple of questions for the author of Moon+ about a year ago and I found his email address, either on his website or on Google Play, I forget which, and emailed him. He replied a day or two later with a detailed and helpful answer to my questions. You might give that a try.
Barry
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