Actually, this doesn't seem to be working, at all. I just created a test game (standard 5E 2024) and tried dropping maps and smaller images on the map layer of varying size pages, and all of them had the Detect Dominant Color slider turned on, and all of them ended up with white surrounds, although the maps I dropped were green, beige, gray or black. What I dropped was mostly older jpegs in my library (which used to set the background color), but I also tried a couple of recent "converted to webp on load" images with the same results. And it didn't matter if I let it fit-to-page or not. I've filed a bug report (#474691).