You can create a custom brush from a preset or from scratch using a raster image.
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 based on the opacity values of a raster image. 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.
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.
Adjust the settings in the dialog. See Modifying brushes for more information.