GIMP is a free program for editing and creation of images, icons, graphical elements of web pages and art for user interface elements.
Its quite advanced, but not very hard to learn. If you have some experience with any version of Photoshop, you will take GIMP really fast. Many of the tools and principles are the same, and work in layers is the main thing. Really recommendable if edititing pictures in Picasa becomes too simple.
The youtube video shows you how to replace a face in a photo.
