Electrostatic decals or window clings adhere with static so you don't have to worry about sticky residue left on your windows.
They are available in white or clear and you can place them inside or outside on windows. There is no adhesive and you can peel them off and use them again and again. if they get dirty, just wipe the electrostatic decal clean. Both sides of the window cling can be stuck, the printed side or the blank side.
Window clings work best on a smooth surface such as glass. Not recommended for Gyproc plaster walls which have a slight ripple texture to them.