One of the keys to building websites, is properly preparing images to help pages load as quickly as possible. Generally speaking, programs like Photoshop, Illustrator, and Fireworks are used, but they can be quite spendy, ranging in price from $300 – $1000.
If what you primarily need to do is crop, resize, and compress images for the web, all of those programs can be expensive overkill. You could use programs that come with your computer, like iPhoto on a Mac, or Microsoft Photo Editor on Windows, but there are other, and sometimes much better, options.