Not sure if some people who're posting suggestions realize that Chromebooks use a completely separate OS, in comparison to the usual platforms of Windows, Mac, & Linux. I dunno about all of those programs, but I know for a fact that Photoshop isn't compatible for Chromebooks.
At least not at this time anyway. When it will be available, as a Chrome app, you'll need a Creative Cloud membership to use it. Older versions, such as CS6 and back, won't work regardless if you can get your hands on a copy of it.
Anyway, my only suggestion is GIMP, but I don't know too much about the program to be honest, since the one time I tried it, it was after I was already used to Photoshop, so I ended up hating it. It's not a bad program and is useful, but I'm spoiled by Photoshop... ^^; If it's compatible with your Chromebook, that's probably your best bet for anything graphics related.