I started fumbling my way through GIMP years ago in Linux. I use it on Windows now, though (which is funny as the program retains a lot of its Linux heritage, and it’s noticeable in places).
CS2 isn’t free. The license servers are offline but it’s still licensed software. Used to use it a bit a while ago; I prefer PS to GIMP, just find it easier to use.
Funny enough I do a lot of basic work in Paint.NET. It’s a surprisingly versatile program so long as you don’t want to do anything complex. Most of my images use a combination of that and GIMP.