Photoshop is, as stated before, the best and most powerful software and is the industry standard. However, you can consider the editors listed here if you don't need all the features of Photoshop. According to what are you going to use the software for, some are better than others...
I think photoshop,
Many friends told me that phoshop is very tuff and not so easy to learn. But i do not agree with them. Because i started photoshop with no knowledge. I started to follow some tutorial by searching google. After viewing 4-5 tutorial photoshop became so easy for me. Now i am very satisfied with photoshop.
And photoshop is the one softaware which can make any impossible possible with images..
If you can get a licensed version of Photoshop then that is really the best out there. There are plenty of tutorials on the net and you will be able to design what you want within a short span of time.
I use Gimp because it is Free and there are lots of Sources where I can find the help I need.
The only problem with Gimp is that it is not so user friendly especially if you are new to any Graphics Design Software