Gimp - Free Image Manipulation Program

An example of the Gimp interface

You can have fun creating Andy Warhol type pictures on Gimp

I've been having some fun with the Gimp program lately.
This free software gives you the tools to undertake many exciting projects.

This excerpt from Wikipedia explains Gimp pretty well.

GIMP (GNU Image Manipulation Program) is a free software raster graphics editor. It is primarily employed as an image retouching and editing tool and is freely available in versions tailored for most popular operating systems including Microsoft Windows, Apple Mac OS X, and GNU/Linux.

In addition to detailed image retouching and free-form drawing, GIMP can accomplish essential image editing tasks such as resizing, editing, and cropping photos, photomontages combining multiple images, and converting between different image formats. GIMP can also be used to create animated images in many formats such as GIF and MPEG through the Animation Plugin.

GIMP's product vision is that GIMP is a free software high-end graphics application for the editing and creation of original images, icons, graphical elements of web pages and art for user interface elements.

There are lots of Gimp tutorials on the internet, try this website for starters http://garmahis.com/tutorials/gimp-tutorials/
Youtube has  video tutorials (I have posted a few below) and there are books available from Amazon .
To get your free Gimp download go to http://www.gimp.org/