Despite his belligerence, Hadrians Hiring! makes an interesting point about the original painting being graffiti - what would the reaction have been if there had been a famous painting of Che Guevara in the town and somebody painted Jesus over it? What would the reaction be if someone spraypainted Jesus on the Kinloch Green changing rooms?
This is not an act of mindless vandalism, it is something else entirely; what it is is not clear.
The original painting is simultaneously graffiti, an artwork, a tourist attraction and a religious icon so the "vandalism" could come from nihilist, artistic, religious or, given the new image, political motivations.
http://www.fnewsmagazine.com/2005-nov/features_2.html
Perhaps it is by someone who has had a bad experience with the church, or someone who took the following quote too literally:
“In fact, if Christ himself stood in my way, I, like Nietzsche, would not hesitate to squish him like a worm”
Che Guevara
Aesthetically it looks to me like something the K Foundation or Trashbat might do, so a "guerilla artist" can't be ruled out.
If the perpetrator is caught, I think it might be quite difficult to a gain a conviction under Scottish Law as Criminal Damage (Malicious Mischief) requires a proof that there have been economic consequences and the painting may lack an "owner" for those purposes?
I'm not sure where the law stands on it being considered as Vandalism and whether political motivation consitutes "reasonable excuse" where religious artifacts are concerned.