Gnu Image Manipulation Program. Very powerful, free, photo edit program. It can do things that Picassa cannot such as correct perspective, filter out artifact, smooth out seams, and bunch of other stuff. Extremely steep learning curve, which is a polite way of saying the program is user hostile. The GIMP people assign whimsical names to things, the on line documentation doesn't describe or explain many obscure concepts used in the program. GIMP enthusiasts claim that GIMP can do everything Adobe Photoshop can do and then some. This may be true, if you have the time to experiment until GIMP starts to work. Version 2.8 can now drive the printer under Windows, something that the previous version 2.6 could not.
Anyhow, wanting to correct the perspective, and being too cheap to buy Photoshop, I downloaded GIMP. And it does work. If someone would write a decent manual, in plain English, it could be a winner.