how to capture a series of plots

Hi, there

I am trying to capture a series of plots into jpeg or png files so that 
I can create gif animation later. However, my plot background generation 
is very slow, and I create the background plot only at the very 
beginning, and at each step I only refresh the foreground points and 
symbols like this:
plot(x,y)
while (condition holds){
symbols(...add=1);
}

My question is, can I create a series of plots test1.jpg, 
test2.jpg...continuously? To my knowledge, R has to close current device 
(by dev.off()) to write to current file, so once first jpeg file is 
written, the current device with background will be off, therefore, I 
cannot add symbols in next device and create following file series.

Any suggestions?

Thanks

WC