As the other CCer said, just leave the frosting sheet in the freezer for a few minutes. When you take it out the picture will peel away from the backing easily.
Just remember to precut any shape before putting in freezer (if you have to). Because if you try to cut the cold frosting sheet some pieces will break. In case if you forget to precut, let the frosting sheet come to room temp. cut your shape and refreeze it again.
HTH
I take a small pallet knife and run it under the image to seperate it from the backing.
Here is a pic of a cake that I did in this way...
http://cakecentral.com/gallery/2141013
HTH
Michael
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