One cannot blaspheme a painting unless one regards the painting as holy in itself, which would be idolatrous to Christians.
This is not to say that I regard the picture as anything but revolting...not because it mocks the substance of my faith, but because it simply dismisses it as something cheap, useful only for padding some fool's pocket.
As Prim has stated, the offense here lies not in insult but in trivialization. And from a Christian point of view, the latter is in many ways the greater offense.