Using your Mac, Linux or windows box, you can automatically manipulate images from the command line using ImageMagick. Most often, I use it to easily add borders to my images. I know I could use a plugin for Aperture, but I often use the “out of camera” JPEG files produced by shooting RAW+JPEG. The best part of the script that I am going to show you is that you can even run it on a web server where your website is hosted to batch adjust images, all in a matter of seconds.
I don’t use windows, so I will attempt to provide the best instructions that I can for Linux and MAC OSX based systems.