[R] Using an image background with graphics

[Waichler, Scott R]

I would like to use a map or aerial photo as a background to plotting
solid lines and text, and semi-transparent color contours, in base and
lattice graphics.  Plot coordinates need to be consistent with the
georeferenced background.  For example, a color contour plot would have
an gray-toned aerial photograph as a background for overprinted
semi-transparent color contours of some spatially dependent variable.
Can anyone point me in the right direction on how to do this?

Thanks,
Scott Waichler
Pacific Northwest National Laboratory
scott.waichler at pnl.gov