I have a cake in the shape of an open book. It is covered in crusting buttercream. I have an edible image that I would like to place on one side. As the buttercream will have crusted,and be smooth,how do I attach the edible image? Do I brush the buttercream with water,piping gel etc?

You can mist the BC with water and then apply the image.
or
Wet a paper towel.
Squeeze out the water.
Lay damp paper towel on BC to soften it.
Apply the image.

I agree with Sally!