I am new to Graphics design. I want to learn a graphics design software in order to develop a game. I chose Gimp as it was free and i followed few tutorials. Most tutorials accomplished the goal without explaining low level concepts such as different blend modes, color terms such as threshold, levels, etc. Where can I find detailed explanations about these? If possible provide free resources. Thanks.
For example at Gimp Documentation
http://docs.gimp.org/en/gimp-concepts-layer-modes.html e.g. has examples of all layer modes
Unless you are asking for more conceptual things, the manual will help with most of your questions.