Do I use a fine spray bottle? What should I use?
Hi, I use a very thin layer of shortening. I have never used water, mainly because I am scared of spraying too much water and that can ruin the edible image.
I always spray the top of the cake with water from a fine-misting spray bottle. As long as you don't touch the edible image with your wet fingertips, it won't ruin it at all. I use half a dozen of them at my bakery every week.
If it's a fondant covere cake just wipe the top with a damp paper towel and apply the image.
For butter cream, I don't use anything...just lay it on a cold, iced cake. Then I continue/finishing decorating..including bordering out the image. It will all mesh/seal itself eventually. For fondant, I do what tiggy does.
In my experience, if you don't spray the surface of a buttercream cake with water to have the image seal itself to the icing, bubbles and wrinkles can and often do appear. All of my cakes come out of a fridge for decorating and go back in the fridge until pick-up or delivery. It would really suck to check on the cakes a few hours later and have bubbles under the image where there were no bubbles before. Water solves that problem completely.
Your mileage may vary.