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The free version of PhotoShop is called the GIMP (GNU Image Manipulation Program). The GIMP has been deliberately sabotaged for years, and years, and years (so that people will still buy PhotoShop) but even with the sabotage, it is still very good (and it is free).
To download a version for your operating system go to https://www.gimp.org/downloads/
If your operating system is Linux, then the GIMP is probably already on your system (look in the menu).
If your operating system is Windows, download the executable (currently gimp-2.8.20-setup.exe) and run it.
If your operating system is Mac OS X, there are DMG installers.
Don't forget to download the GIMP's local (offline) help in your language (e.g., gimp-help-2-2.8.2-en-setup.exe for Windows users who read English).