They used a frozen buttercream transfer to make the image.
It's a process where you outline over a picture (that has been printed out in reverse) on wax paper with buttercream icing. You then freeze the image so that it hardens and stays together, then after you have frozen it and you have finished icing the cake, you take the image out of the freezer and quickly flip it onto the cake. Remove the wax paper and voila, you have a frozen buttercream transfer.
In the forums you will see the frozen buttercream transfer referred to as BCT (or just buttercream transfer). Cali4dawn has a wonderful tutuorial on her site and this site on how to do it... http://www.cakecentral.com/article12-How-To-Create-a-Frozen-Buttercream-Transfer.html