Dynamic Desktop wallpaper thumbnails
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.