Yes they can be put on fondant, gumpaste, and buttercream.
Here is a cake I did that I put them on fondant.
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1397584
Here is one where the pictures were put on Gumpaste a couple of weeks in advance and I let the gumpaste (with the picture) dry so I could stand it up on the cake.
http://cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1397586
I LIGHTLY mist the fondant before placing the image on the cake if it has gotten a little dry (which on the betty boop one it did because I covered all of the tiers first then did all of the photos)
For the gumpaste I rolled it out to the thickness I wanted and added the photos while it was still fresh and it had not gotten a skin on it yet.
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