To understand and experiment with digital image processing, photo correction, photo manipulation, and photo montage techniques.
Digital image processing is the use of a computer application to transform the characteristics of a digital image. The process could involve any number of transformations including changes in hue, value, brightness, etc.
Adobe Photoshop in particular has become the leading digital image editing application for the Internet, print, and other new media disciplines. It is embraced by millions of graphic artists, visual communicators, and print and web designers. Traditionally, Photoshop has been and continues to be a print industry standard. It's likely that the majority of pictures you see on a daily basis(such as posters, book covers, magazine pictures, and brochures) has either been created or edited in Photoshop. The powerful tools used to enhance and edit these pictures are also capable for use in the digital world including the infinite possibilities of the Internet. In the past few versions, Photoshop has been accompanied with ImageReady. This sub-program is specifically made to deal with web applications including slicing Web images, producing rollovers, supporting enhanced image maps, and compressing graphics files efficiently for the Web. It is almost impossible to use or even learn all of Photoshop's tools to their maximum benefit. Photoshop has been around since 1982!
--excerpted in part from:
"Photoshopping" or just "Shopping" is slang for the digital editing of photos.[The term originates from Adobe Photoshop, the image editor most commonly used by professionals for this purpose; however, other programs, such as Paint Shop Pro, Corel Photopaint, Paint.NET, or the GIMP, may be used.] "Photoshop" is widely used as a verb, both colloquially and academically, to refer to retouching, compositing, and color correction carried out in the course of graphic design, commercial publishing, and image editing. In popular culture, the term photoshopping is sometimes associated with montages in the form of visual jokes, such as those published on the fark.com website and in MAD Magazine. Images may be propagated memetically via e-mail as humor or passed as actual news. An example of the latter category is "Helicopter Shark," which was widely circulated as a so-called "National Geographic Photo of the Year" and was later revealed to be a hoax.
--from Wikipedia: http://en.wikipedia.org/wiki/Photo_manipulation#Photoshopping
--Williams, Alex, "I Was There. Just Ask Photoshop," New York Times, Aug. 17, 2008 (link)