There are free alternatives to Photoshop. My favorite is Paint.NET. With this software you can do layers and adjust curves. Find some free plugins and the possibilities are almost endless. 95% of photographers will find that Paint.NET can do everything that they need a photo editing software to do.
|Roadway - Tehachapi, California|
Post-processed using Paint.NET.
|Summer Mow - Tehachapi, California|
Post-processed using GIMP.
Another free photo editing software that is surprisingly powerful is Picasa. It is not for everyone, but for those that just need something that is easy-to-use, this is it. I would bet that 50% of photographers would find that Picasa is all that they need.
|Pathway To The Soul - Tehachapi, California|
Post-processed using Picasa.
I believe there are two reasons that most photographers buy Photoshop instead of using these free programs. First, most don't know that free alternatives exist. It is mostly by word-of-mouth that Paint.NET and GIMP are discovered. Second, Photoshop and those that sell it have done a great marketing job, convincing people that they need it when they really don't.