User-Defined Brush
The User-Defined Brush tool is similar to the Standard Brush tool but with the ability to control more of the brush's properties. In addition to changing the size and style of the brush strokes, you can also modify the opacity, fade, density and movement of the user-defined brush.
To use this tool...
After selecting the colours you wish to paint with, Click on the User-Defined Brush button and point the cursor where you want to begin painting. Hold down the left mouse button to paint with the foreground colour; hold down the right mouse button to paint with the background colour. Paint with the brush by moving the mouse. To change any of the brush's properties, double click on the User-Defined Brush button.