Can someone tell me. . . . when doing a FBCT and you are done doing all the filling in of details, is it absolutely necessary to cover the entire image with buttercream before freezing? Or can one do the image without covering it, freeze it and then put it on the cake? The reason that I am asking is because I am doing a FBCT to put in the center of a sheet cake and would like to do it without the icing backing (even if the backing is the same color as the rest of the cake's buttercream) to make the image lines look sharper. Can someone help me. . . .
I believe the backing helps stabilize the image. If you have time, you could try a partial image and see how well it will transfer to a test surface, like a sheet pan. If the image is fairly thick and very frozen, it might be OK without the backing. but I would definitely experiment.
i agree with joanneb. should work if there are no empty spots and the image is good and thick. experamentation is always a good idea.
I've done 2 FBCT without using the backing, and no matter how long they were in the freezer they broke when I went to put them on the cake. I was able to piece them back together nicely, but I think next time I will put a thicker back on it .
I dunno about you ladies but the first time I did a FBCT it didn't work at all! I did everything the tutorial stated to do I had it pulled up on the laptop the entire time sitting there doing it, I went to put the "backing" on it and when I did it pulled all the icing up and out of their spots and mixed it all together and turned the white backing into a rainbow of beautiful colors. I mean it was pretty and all but darn it it didn't work for what I needed lol I froze it anyway and it still didn't turn out right when I turned it over onto the cake it had major gaps in the colored icing where the backing pulled it out and mixed it in with the backing 
The first FBCT I did I forgot to add buttercream to the back and it fell apart when putting it on the cake. You can see in my photos(the Marine Corps cake) where I had to piece it together. I did one last week and used the big icing tip to add the buttercream to the back and it worked great. I would definitely add the buttercream to stabilize it.
I'm assuming that you don't want the image raised like they normally are. If that is the case, you can do the whole top of the cake as the image.
What i do, is trace the cake pan, then trace the image in the outline, how i want it on the cake. Then i fill in the image, then using the icing tip fill in the rest with the ridge side up. Then when frozen, I continue the cake with the upside down icing technique
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