In my level, I want to use a version of Beam Tileset (no. 57) in which a couple of the mirrors and beam shooters have the beams turned off. The description bar would say something to the effect of "Beam Tileset - Original by Joshua Green - Modified by ExpHP".
It appears that these tilesets are free to use in their original form, but I don't know the permissions for derivative works, so I'm not sure if I will need to get permission from this Joshua fellow.