I want my local bakery to print an edible image for me to apply to a cake.
Can I apply that image to a findant covered cake? Or does it have to be Buttercream?
Also, how easy is it to apply? And how far in advance can I do it?

I have applied images to fondant as well as butter cream. I would not refrigerate the cake though after applying image to fondant. Fondant tends to "sweat" after being in the fridge causing the image to dissolve some.

The image only needs moisture to adhere. So if you use a crusted buttercream, be sure to remoisture it. If applying to fondant, make sure it is moist. If applying to fondant and want to refridgerate, do not uncover the cake until it has come to room temperature so you can avoid the sweating problems.