R plots in PowerPoint

[Jamieson Cobleigh]

I have some plots that I generating using R (using trellis, plot, and
barplot2) that I want to include in a PowerPoint presentation I am
giving.  My computer is running OS X.  I find that presentations are
easier to read when there is light text on a dark background and would
like to generate plots that fit that style.  The plots I am generating
are all grayscale.  Is there an easy way to get R to "reverse" the
plots, to make black into white and white into black?  Also, is there
a way to have R generate images where the background color is
transparent?

Jamie