To avoid air bubbles, when you apply the frosting sheet you should pat it down to remove the air bubbles from underneath the sheet, and then leave the sheet for 30 mins to allow the printed image to absorb into the cake's icing.
The frosting sheets should be printed to at room temperature, but putting the printed sheets into a DRY freezer (not wet freezer) for 10-12 seconds will help in making them easier to peel off and lessen the chances of air bubbles. And ensure that youre placing the printed sheet on a low moisture, buttercream type frosting on the cake. If the coating is not buttercream like, but rather is a higher moisture type like whipped cream, you should put a thin layer of buttercream on top of the whipped cream, and/or coat the back of the frosting sheet with a thin layer of butter-cream before laying it on top of the whipped cream surface.