Photoshop brushes fall into two categories: paint-type and image-hose-type. A paint-type brush mimics the texture and behaviour of a real painting tool, such as a paintbrush, a sponge brush, or a paint roller. In contrast, an image-hose-type brush sprays a single graphic element onto your canvas, allowing you to vary its size, shape, and colour.
Be aware that a brush tip is a greyscale image. A colour image made into a brush tip will be converted to greyscale, and the grey values will be partially transparent; this transparency will carry over to brushstrokes. This means that any part of the brush tip that is black is fully opaque, and will yield full colour when used as a brush.
Also, be conscious of brush size. If your brush tip is 500 x 500 pixels, you can scale it down without any loss of quality; however, if your original brush tip is only 25 x 25, scaling it up to 500 x 500 pixels will look terrible.
The source image for a brush tip can be just about anything. Here are three examples of image types you can use:
Spongy Brush Using a loofah sponge, dab black paint onto a canvas. After it dries, scan or photograph the texture and bring it into Photoshop. Convert the image to greyscale (Image > Mode > Grayscale) and tweak the levels (Image > Adjustments > Levels) so that white areas are free of texture and black areas are pure black.
Wheat Stalk Brush Use Adobe Illustrator (or another vector drawing app) to draw a single, detailed stalk of wheat. Do not use any colour; the stalk itself should be black and the background transparent. Import the final artwork into Photoshop.
Flower Brush Open a photo of a flower in Photoshop. Convert the image to greyscale, and then mask the flower, if necessary. Tweak the levels to add more contrast to the flower. Also make sure none of the flower areas are too light.
Once you’ve made the necessary edits, you may need to scale the document down. Choose Image > Image Size, enter a maximum width or height of 500 pixels, and click OK. Choose Select > All, and then choose Edit > Define Brush Preset and type a name for it. The brush head will be added to your current set of brushes.
Save the document and close the window. Now choose File > New to create a new document that will serve as a scratch pad for the next step.