Make sure that the folder you copy opens directly to the image files themselves.
Open the game (may take a while) and load a save.Linux ~/.local/share/Steam/steamapps/common/Hollow Knight/hollow_knight_Data/Managed/Mods/CustomKnight/ Mac ~/Library/Application Support/Steam/steamapps/common/Hollow Knight/hollow_knight.app/hollow_knight_Data/Resources/Data/Managed/Mods/CustomKnight/ Windows C:\Program Files (x86)\Steam\steamapps\common\Hollow Knight\hollow_knight_Data\Managed\Mods\CustomKnight\