Let's see how to save your own custom brushes *Ü*. I'm using my sunflower:

Place the image or the tube in a new file. Be sure you don't have extra space. If you are using a tube, the best way is to copy it and past as new image. The canvas will fit exactly the image:

Go to File/Export/Custom Brush. This window will open. Name and describe it if you want, choose the path where you have your brushes, and let the step in 32:

Go to your brush tool, and you'll see the new brush there. See how it looks:

Nice! You can change the opacity of the layer to get a shadow effect:

Or some levels of shadow... These are good for a stat, don't you think?

If you like this brush, you can download it HERE.