If you already have Photoshop, then GIMP won't really offer anything more or vastly different.
If you want to resize anything in multiple, then setting Automator would be your best bet.
I know Macromedia Fireworks lets you do batch processing. There must be a way to do it in Photoshop too. I'm a PS nOOb so I can't help you there, but I would think that if Fireworks can do it, PS should be able to.
I recommend going the Automator route, just google for Automator tutorials as I'm pretty sure I've seen one somewhere that does exactly what you want.
The reason I recommend this is that learning Automator for simple batch tasks like this will help you unleash the power at your fingertips, you will suddenly find out just how powerful a computer can be when you know how to make it do things your way instead of continuously hunting for tools that suit your needs.
Amen-Moses
(of course this assumes you are running Tiger, if not you will probably need to use AppleScript instead, there are plenty of AppleScript tutorials out there anyhow.)
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