Oh! I can answer that one!
I mean is makes sense in hindsight, if you combine your knowledge Hitlers prior career as an struggling artist, with Meta knowledge of Tropes like "The Picture of Dorian Grey" and "Doctor Who"
Honestly I stumbled on it myself by virtue of being a completionist. I stumbled on a painting by Hitler somewhere, and it was bad, really really bad, and I passed it off as an Easter Egg. Then I found another painting by Hitler, and it was also really bad, but it was a different picture then the first one. So I saved and spent the energy for a world wide scan looking for more. Found a few more, each one unique. At this point I knew something was up, because there was no collection of Hitler paintings being tracked in the album, they were not grouped together in inventory if you collected them, could not put them in the Trophy room, no "Hitler Paintings Viewed" stats section, nothing. Yet some artist rendered a unique picture for each and every painting that only appears once in the game. I was intrigued.
So I went and started a new game, and that OL apparently thought, "So, I am stranded in a strange dimension, obviously the very first thing I should do is to scan for paintings by Hitler." And there were some new ones I did not know about! Someone was warding Hitler Paintings in my old game! Now I was a god with a bone. I kept the new game going, and did something I had never done before, avoiding the "Shine a light on the Shadows" and all similar quests, as gaining reputation points from those quest lines makes your opponents ward against you) but went ahead and researched the "Ward busting" tech tree in Atlantis any way... If you get near the end without doing anything to trigger the ward upgrades, AND upgrade your Ring Analysis Tree, you can find a Painting by Hitler in the Louvre! A Good painting, not the crap every single one of his other paintings looks like, which is why you need to upgrade the Analysis Tree, so the Ring can identify it.
So that is how you can find out about it. There are also some clues in the Muse Loyalty Quest lines. If you steer the conversation in the right direction while the Muse is wearing a Spell Eater (you should give one to everybody, just good practice) she reminisces about Hitler like he was a good artist!
Outside the game, if you look at the Muse Trophy achievements, the flavor text is also a hint. Basically, Hitler did used to be a talented artist. And a dabbler in the occult. And a worshiper of the Norse. So he used the Spear of Destiny, and the Ritual Odin used to sacrifice his eye for knowledge, to sacrifice all his Artistic fame and talent, to create an inevitable destiny of power for himself. That last good painting of Hitlers is all that remains of the old timeline, and in fact contains that timeline within it. As long as it exists, Hitler's rise to power is a fixed point in time, and cannot be changed.
And there in the Rub. The painting can only be destroyed from the inside, and getting there requires you perform a Quintessence Waveform Scan, and enter through the Honden of Avarice (Timelocks are hard to break), Which needs the Ophidian. But the Ophidian quest line makes enemy wards on the Painting so strong, you will never break through with a scan to find it.
Basically it is new game plus content. You need to find out about it in one play through, and actually fix it another. That is why is seems like a Guide Dang It!
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