photoshop advice for a project?

So I am needing some photo editing software for this project: I am going to be taking dog pictures. I will need to preserve the original. I need to crop at different places. I will need just the head. Then for a second picture, just the body, and the third just the tail. So for all types, the original, head shot, body shot, tail shot, I will need to have a background that is the same. I will be using other dogs, so it is important that the background is the same for every shot every dog. I need to be able to change the background using the software. I know photoshop is supposed to be good. I have looked up the CS5 extended version for students and teachers. Can anyone tell me if this is a good product for the editing that I will need? Will I be able to do everything that I mentioned? Also, what kind of info will they need to verify that I am a student? Any information about the helpfulness of photoshop and the ease of using it would be appreciated. Also, if you have any information about another type of photo editing software let me know. Thanks so much!