Use
any selection tool to select a part of an image to use as a custom
brush. Feather should be set to 0 pixels if you want to create a
brush with a sharp edge. The brush shape can be up to 2500 pixels
by 2500 pixels in size.
If you select color image, the brush tip image is converted
to grayscale. Any layer mask applied to the image doesn’t affect
the definition of the brush tip.
If you want
to define a brush with soft edges, select pixels with gray values. (Colored
brush shapes appear as gray values.)