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I have downloaded various "Dynamic Desktop" wallpapers online, put them into a directory, and added this directory to my Desktop & Screen Saver preference pane.
The problem is, it does not correctly the thumbnails. They all appear to be the "Mojave" wallpaper, while in reality they each are different pictures.
On Mojave the system doesn't generate correct thumbnails. The only workaround is to place the wallpaper into /Library/Desktop Pictures/ and place its thumbnail into /Library/Desktop Pictures/.thumbnails/ folder. This requires administrator password and can only be performed from terminal.
sudo cp mywallpaper.heic /Library/Desktop\ Pictures/
sudo cp mywallpaper_thumbnail.heic /Library/Desktop\ Pictures/.thumbnails/mywallpaper.heic
Thumbnail has to have same name as wallpaper. macOS Catalina can create correct thumbnail. Custom thumbnails are only supported for wallpapers inside Desktop Pictures folder.
