I'd use 'graphical' when referring directly to the picture, etc,, and 'graphic' to refer to the area, subject:
A graphical representation
A graphic designer
Graphic- this can also mean full of colourful and usually unpleasant details- a graphic description.
Use and usage- it depends on the context. 'Usage' is generally for specific situations- a langauge form used in a certain area or by a certain groupd would be a usage.