You want to create a bitmap of your plot, writing to a PNG file.

There are two ways to output to PNG bitmap files. One method is to open the PDF graphics device with png(), make the plots, then close the device with dev.off(). This method works for most graphics in R, including base graphics and grid-based graphics like those created by ggplot2 and lattice:

# width and height are in pixels
png("myplot.png", width=400, height=400)

# Make plot
plot(mtcars$wt, mtcars$mpg)

dev.off()

For outputting multiple plots, put %d in the filename. This will be replaced with 1, 2, 3, and so on, for each subsequent plot:

# width and height are in pixels
png("myplot-%d.png", width=400, height=400)

plot(mtcars$wt, mtcars$mpg)
print(ggplot(mtcars, aes(x=wt, y=mpg)) + geom_point())

dev.off()