To get real coordinates for the pivot point use:
def fn(Input):
import arcpy
myRaster = arcpy.Raster(Input)
ext = myRaster.extent
midx = ext.XMin + ((ext.XMax - ext.XMin) / 2)
midy = ext.YMin + ((ext.YMax - ext.YMin) / 2)
return "{0} {1}".format(midx,midy)