To the commenters blindly recommending Sketch.. May I please point you to this part of the question? Thank you very much.
And I also Run on a Linux PC
Sketch is a Mac app.
To answer your question: If you want to go with Photoshop, you can get a cheap 1-app subscription at adobe.com that'll cost you like €12,- a month. Otherwise you could go for free graphic manipulation programs like Gimp (horrible interface, great power) or any of the other apps @xananax recommends here in the comments.