Answer:
There are many free applications out there. For Windows, my favourite is <a href="http://www.onlinetools.org/tools/zips/easythumbs.zip">EasyThumbs</a>. It lets you resize many pictures at once if you need to.
For Mac, you can use Automator, a program that comes with Mac OS X. Here's an article about how to resize pictures with Automator: <a href="http://atkinsfamily.org/2008/01/resize-images-using-automator-and-mac-OS-x/">click here</a>.