jpeg or tiff file formats

I have been creating some graphic postscript files using xyplot but now 
I find that I need then in either the jpeg or tiff format.  I have tried 
re-running just the lattice plot so that it is on the screen and then 
using the jpeg(filename="myplot"..... from the "bmp, jpec and png 
graphics devices" documentation. It seems to create an empty file. Is it 
that this function can not work with lattice plots? Can R directly write 
a jpeg or pgn file?

Dean Sonneborn, MS
Programmer Analyst
Department of Public Health Sciences
University of California, Davis
(530) 754-9516