I'm not familiar with this in R, but I suppose a quick and dirty way would be to flip the matrix itself that contains the data... If your data is in a matrix or matrix-like structure, I suppose you may be able to flip it left-to-right or right-to-left, similar in concept to Matlab's flip up-down function...
Assuming your data that contains how the skull is to be rendered looks like this:
0 1 0 1 1 0 0 1 1 0 1 0
0 0 1 0 1 0 => 0 1 0 1 0 0
0 0 0 0 1 1 1 1 0 0 0 0
Can you perhaps provide more details on the problem, like sample data and more sample code snippets? It's hard to tell without those...