Adobe Photoshop is definitely the best program to use for editing photo or image files. iPhoto also has some options, but it pales in comparison to the control you get with Adobe.
Adobe Photoshop
Adobe Photoshop CS4 is the latest version of this program. You can buy it from http://www.adobe.com/. OR... you can try using this instructional video to download it for free. If you try this, Adobe Photoshop CS4 will work absolutely perfectly. It won't expire or be limited if you use this option. All you have to do is go to the adobe website and download the trial version of Adobe Photoshop CS4, which will expire after 30 days. Then download the crack, which can be found in the rapidshare links in the description of this video, and crack the trial. The instructional video can be found here.
