1. Open png file
2. Resize so that the file is no larger than 500 on either the width or the height.
3. Click on your "Brush" tool
4. Once the Brush tool is selected, you will see a brush shape on the top tool bar in a white box. Click on the down arrow key next to it. A screen will drop down (all of your brushes will appear; this would be the area you would choose a different brush).
5. On the bottom left of the little screen, there is an icon that looks like a brush with a hatched box around it. If you scroll your mouse over it, it will say "create brush tip from selection" - click on it.
6. A new box will appear in which you must name it, etc. Push Ok.
7. Go back to the brush tool & you should have it there.
had directions and photos on how to do it. It's a lot easier looking at the screen shots while doing it.
I hope this helps!