Photoshop is available for Mac, but they've switched to a horrible rental system which gets expensive.
Photoshop Elements is a more economical offering, but the version I tried proved to have a completely different philosophy and I never got to grips with it after the nice intuitive interface of Photoshop 6 (it was awhile ago).
I opted for an app called
Affinity which seems to be highly regarded by those in the know and is a bargain compared to Photoshop CS. It looks powerful, though I've not had time to get used to it... all I can say is it has quite a steep learning curve!