A Brush stamp is just an image imported into the brush editor, then repeated along a drawn line according to settings you make. You can import an image straight from the brush editor, from your camera, photo roll or Files / Cloud.
We recommend you use a 256 x 256 px image size for smaller brushes and a 512 x 512 px image size for other brushes, depending on your needs. We do not recommend images larger than 1024 x 1024 px (you might use these for giant background-style strokes or very large prints). The larger the image size, the more performance and memory it will take up on your device as it repeats in the stroke.

For help creating your own stamps and grains to size, or if you just want some examples to get started using the brush editor, read this article here. It’s actually quite easy.