If you have for example like mine - a tree to paint around, why not export the image with no trees. Do your painting and then export an image with just your trees and overlay that on top of your 'painted' image.
I was going to say you can use a texture (such as a brick pattern) for your brush. I guess the trick there is trying to match the perspective or orientation of the brick on the face of the building you are painting on with the brush. If that makes sense? haha.
