You can create a custom brush from scratch using a raster image, from a copied preset or from a pixel selection.
To create a custom preset brush stroke from scratch:
On the Brushes panel, click Panel Preferences and then select:
New Intensity Brush—creates a brush stroke using a raster image, with darker pixels from the image rendered more opaque and lighter pixels more transparent. In the pop-up dialog, navigate to and select a file, and click Open.
New Round Brush—creates a brush stroke based on a circular shape.
New Square Brush—creates a brush stroke based on a rectangular shape.
New Image Brush—creates a brush stroke based on the color values of a raster image. In the pop-up dialog, navigate to and select a file, and click Open.
The new brush is added to the selected category using default settings. To edit the default settings, follow the procedure below from step 3 immediately below.
To create a custom preset brush stroke from a preset:
On the Brushes panel, select a brush and click Edit Brush.
In the dialog, click Duplicate and then Close.
Select the new brush at the bottom of the panel and click Edit Brush again.