In the Cascade Kingdom - Fossil Falls section of Super Mario Odyssey there is a floating island directly above the Top of the Big Stump waypoint.
I found the painting. The one in the snow kingdom leads to the cascade kingdom. I think the issue lies in the fork in the road when you finish the metro kingdom. You can go to the seaside or the snow kingdom. If you go to seaside first, the painting takes you to cascade and the snow one takes you to the lake kingdom. If you go to the snow kingdom first, it is the opposite. I went to snow first, so that's why it takes me to cascade and the seaside painting takes me to lake. This should solve the disparities. The painting in the snow kingdom is behind a tall wooden block near the odyssey that you can blow away with the cloud enemy near the odyssey. I would show a picture but it won't upload, so I hope this helps
According to Polygon:
To get this power moon, you need to travel to Seaside Kingdom. After you beat the boss there, swim and climb up to any of the water spouts. They’ll fling you to where the boss used to be. Swim down from where you land, and you’ll find a painting. That will warp you to an island overlooking Cascade Kingdom. There, you’ll find a flag and a power moon.
I've definitely not made it to Seaside Kingdom yet myself, but their screenshots seem like that's definitely the one.