The GIMP (The GNU Image Manipulation Program).
It has many of the features that Photoshop has, but you don't have to spend hundreds of dollars for, since it's an open-source program.
I suggest you give it a go.
You need to install two parts.
Firstly:
GTK+ 2 Runtime Environment (version 2.8.18, for Windows 2000 and newer)Once that is installed:
The GIMP for Windows (version 2.2.11)There are also version available for Linux and MacOS so everyone wins!
Once installed and running, browse some of the
Tutorials. There are many indepth examples to get you started, complete with examples and screenshots.