In GIMP (free photoshop) I would make a new layer and print the brush on the layer (preferably transparent background) then select by color the back ground and move to the layer with your image invert the selection (in gimp it's select > invert) then cut (then make the brush layer invisible or delete it)
GIMP is great if you can't figure out how to do something in Photoshop and, if you had that, you could contact me any time for help using it.