I don't know if any of them are really easy, but there are some free ones out there.
1. Paint (comes with Windows) Very basic program.
2. Gimp -- I haven't used it much at all, because I have PhotoShop, but it looks like it would do just about everything.
Main Web site:
http://www.gimp.org/
Windows downloads:
http://gimp-win.sourceforge.net/stable.html
You need to download two programs to install it, one that's the program, and the other installs a "runtime environment," which you need for everything to work right.
(They also have versions for Mac and Unix.)