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The free clone of PhotoShop is called the GIMP (GNU Image Manipulation Program).
If your operating system is Windows,
then go to http://gimp-win.sourceforge.net/stable.html
and download the executable suitable for your language and system.
For the 32-bit English language version the executable is called gimp-2.6.8-i686-setup.exe
Then just run the executable.
If your operating system is Linux,
then the GIMP is probably already on your system (look in the menu).
If not, install it from your installation disks (or get it from the internet).