I have Photoshop CS4 from Adobe. I am working on my masters in educational technology and I will tell you that it is a hard program to learn. If you decide to go with Photoshop prepare to pay a lot and watch a lot of tutorials. It is doable but don't plan on being able to do it as soon as you open up a new document.
GIMP is a free version that is sorta similar to Photoshop. You may try it first then decide if you want Photoshop or not. Here is a link to download if you are interested. Good luck!
GIMP - The GNU Image Manipulation Program