I don't think it's strictly legal to download photoshop. After all, it is copyrighted software and there's a whole lot of laws that protect programs and software developers against that. I mean online even most images are copyrighted and you can get in trouble for using them without permission, so I imagine the penalties with something like Photoshop are a hundred times worse.
Seeing someone mentioned about GIMP as free PS, I also come up with other freeware as Photoshop substitute, like: Ifranview, Fotor, Fixlr, Picmonkey....Ifranview is less complex than PS, the other three is even simpler than Ifranview. So some function of PS can't find there.
No doubt photoshop is the best editor in this world. But you should also check a Photoshop Online Free which is same like photoshop express but has some advanced feature, if you are looking for free one.
Paid one no one in the world can compete with photoshop.
The older version of Photoshop is not legal.
They published the software installation files and keys on their site, when there was an issue with sending the keys to the people who purchased the software (as i remember).
You can try Gimp, I heard you can customize with the add-ons, so it will look like photoshop.