Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
The File>Export option in iPhoto might be what you want, depending on how much control you want to have over the image. Otherwise, I use GraphicConverter myself, well worth the (low) cost, which does much more, and allows you more fine-grained control when resizing.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.