Preview is the default image editor for your Mac. Even though it's shipped along with your Mac for free, but it's surprisingly useful for Mac users, especially for those who have upgraded to Mac OS X Leopard.
First, I will guide you to learn the very first and important technique of using Preview application, i.e. opening multiple images at once in one Preview window.
Before you opened many images file with your iPhoto, think first. You will give heavy load to your Mac. It's better for you to open all of those images file with Preview application.
When you're going to open many images at once, don't forget to set your Preview Preferences (Command-Comma) to open all selected images in one window (if not, all images will be opened in separate windows).