One of the advantages of having so many painting apps, is the unlimited supply of dabs to create custom brushes in #Photoshop. Most will let you share in the PNG format. If not, the magic background eraser tool in Photoshop can do it one click. If not removed, the background becomes part of the brush. Not so nice.
In #ArtRage 4, you can also import your Photoshop brushes from the Sticker Spray tool.