hi john,
did i get you right, you want to define your own brush?
– select the desired part of your image with the marque tool (you can also crop it)
– copy “Ctrl+C” the area
– open a new document “Ctr+N”
– paste the selection into it “Ctrl+V”
– choose “edit” “define pattern” or “define brush”
– choose a name
mention that you cant use the brush in a bigger size as you created, without scaling.
hope thats what your looking for, greetz