Adobe Photoshop is essential for processing digital images. I am a Linux programmer/user, and would love use the Linux GIMP application, which is available for Windows, to edit my photos, but I rely too much on the "Shadows & Highlights" feature of Photoshop CS2. Adobe also has sophisticated Image Sharpening and Noise Reduction and I am not sure GIMP can match it.
So, you want to get Photoshop and spend as little as possible. First, Photoshop does security by Activations. When you buy a license, you receive I believe two activations. Should you loose a computer, you loose an activation. If you reinstall a computer, your activation is
normally preserved. If you have extenuating circumstances, such as two stolen computers (my situation), Adobe will give you a third activation. It is important to know about activations when purchasing Photoshop used; make sure you find out as much as you can about how many activations you are getting.
What version to get? CS2 has "Shadows and Highlights" so it is sufficient for me. My workflow consistes of Levels, occasionally Curves, Noise Reduction, Sharpening (Smart or Unsharp Mask), Blur (Gaussian), Shadows and Highlights, and other basic operations such as crop, cut, and paste. Everything I needed was in CS2, except I think that alternate Magic Wand tool which became available in CS3, and is somewhat useful.
Adobe offers a student discount. You have to send your documents to Adobe before receiving a serial number. On Amazon.com you can buy Photoshop new and used. Be careful buying it used, but keep in mind that Amazon will support you in case of seller fraud. Here are some links.