OT: graphic designers shop knowledge required..

It's probably easier to do it by making a droplet (essentially generating an app-style icon that you can drop image files onto), then you don't need to open all the images in photoshop in one go (something that will most likely cause your computer to crash, unless you have a beast or lots of time on your hands).. Or if you have bridge, you can cue op batches from there and go put the kettle on.

Not going to write a tutorial but google brought you a present: http://www.photoshoplab.com/droplets-for-the-lazy.html

No need for a droplet, no need to open all the images, just batch them with potatoshop's built in 'batch'.