Most people on here use Photo Shop Plus. There are several versions of it and the older ones you can find online free.
Do a search for Photoshop and see what all you can come up with. There are some others too. And there are online tuturals for them also. I'm still learning. But there are others on here that are much better than what I am. I just picked this up for a hobby to fill in time as I'm an older lady and live alone. But it is something fun to do. There are programs online too that you can get a 30 day trial with. Can always get one of those also to play around with and see if you really like it. Zet aka Suzette use to have sites listed on her profile of pages to teach one how to use psp (Photoshop Pro). She has been on GG for long time and is one of the moderators here. So you might want to ask her also... Hugss n take care :)