Invisible Item Frames
Item Frames can be made invisible by throwing a Splash or Lingering Potion of Invisibility. You can revert the invisibility using a Splash Water Bottle, or by just breaking the item frame.
For lagbusting reasons, Item Frames require an item in them in order to be turned/remain invisible. Invisible Item Frames will become visible if an item is removed, but turn invisible again if an item is re-added.