Being a Photoshop professional (I make my living with it)...I'm always going to go with PS. But...if you are only going to use it on rare occasion and not be editing images to the degree of professional quality....then I'd say you're not going to want to spend the money for PS. It is expensive.
I can't really say that I've had experience with any of the free software that is mentioned here, but in PS, you can do anything. Those that don't use PS in the capacity that I and other professionals use it really don't have a handle on what really is possible within the program. I've been told that the most proficient users of PS only use about 45-50% of the programs capabilities. I can totally understand that.
If you have the money, and the needs.....you can't go wrong with Photoshop. Good luck.