imagemagick

Do you use the tool “convert” to manipulate plots and images?  It’s simple, powerful and downright great!  Quick examples:
Modify a plot for presentation use, by swapping to a black background with white lines, so it’s easier to read on a screen:
> convert  -negate frompaper.ps  forscreen.jpg
Make a thumbnail:
> convert -geometry 50×50 big.jpg thumbnail.gif