As far as I know, you have to purchase Adobe Photoshop. I have used Photoshop. It can be a little pricey.
I haven't used Corel Paint Shop. It is about half the price of Photoshop. I have heard good things about it though.
Have you heard of Fotoflexer? It is a free and might do what you are looking for. Here's a description of it. See what you think.
http://news.cnet.com/8301-17939_109-9766535-2.html