apply gentle heat, perhaps a hairdryer for a short period or put it on a baking tray in a warm oven (not hot) for a couple of minutes, or a fan oven with the fan on but the heat off, or hold it under lamp with a warm bulb.
try steam over a boiling kettle or gently spray a fine mist of water over the top of the edible photo.
you will get better results if you peel the backing away from the icing, rather than the other way round.
carefully remove the frosting sheet from the plastic backing and apply it to the iced cake by lowering the icing sheet down onto 