Well, I have GIMP, but maybe there is something like this for photoshop too...Anyway, here is what I do:
You see there are actually three brushes on one image? Well, open a new image, large enough to hold one of the brushes, make sure you convert the image to grayscale before going on. Then go Script-Fu>>Selection>>To Brush
I'm not sure if that s how it works i photoshop, but that's how I do it in GIMP, hope that helped
