[Cassie Jones (1gr8muggle)]

Brushes have to be smaller than 2500x2500 pixels. (about 8x8 in.). Basically, if your png is smaller than than, you can just go to Edit > Define Brush Preset, and you'll have a brush.

I have a detailed tutorial on the process here:



Even though it uses a photo as the source of the brush, all the ideas are there that you would need to make a brush from anything.

Hope this helps!