The same can be said about a painting of Satan at the entrance to a school.
Nope. Legal precedent says otherwise. That's why a school can't put a cross or Star of David on its roof and argue "There's no message there, so it's fine".
County of Allegheny v. American Civil Liberties Union
"the majority held that the County of Allegheny violated the Establishment Clause by displaying a crèche in the county courthouse, because the "principle or primary effect" of the display was to advance religion within the meaning of Lemon v. Kurtzman, when viewed in its overall context."
Just like the Jesus painting, when it's by itself with no other context, it is unconstitutional.
You need to learn to read, the final decision:
A different majority held that the menorah display did not have the prohibited effect of endorsing religion, given its "particular physical setting". Its combined display with a Christmas tree and a sign saluting liberty did not impermissibly endorse both the Christian and Jewish faiths, but simply recognized that both Christmas and Hanukkah are part of the same winter-holiday season, which, the court found, has attained a secular status in U.S. society.
Also the problem with the nativity is that this was enclosed with it:
Gloria in excelsis Deo
Alone, it would not have been an issue, like i said, the message with it, was the endorsment.
There is no message with the picture of Christ.