init python:
     def phoebe_func(st, at):
           filename =  "{}_{}_{}_{}.png".format(
                hairType,
                colorHair,
                colorEye,
                skinColor,
                )

           return filename, None

image phoebe = DynamicDisplayable(phoebe_func)


label start:
     $ hairType = "short"
     $ colorHair =  "red"
     $ colorEye = "green"
     $ skinColor = "white"

     # Uses short_red_green_white.png.
     show phoebe

    "Phoebe" "Hello, world."