I think you shouldn't post anything that might incite a religious debate.

e.g. Submitting a cross might be allowed but submitting an upside down cross may not be.

(I am not an admin all of above are just speculations.)

the terms of use state that submitted art must not be offensive, what is offensive is at the discretion of the site admins.   There would be leniency on most art, since we already have plenty of demons and demonic iconography.  The rule is mainly to keep threads from descending into flame wars. 

I posted a replica of Moses' Tabernacle without incident, but I'm sure a depiction of Jesus slaying those with the mark of the beast my upset some people in these times.