good question!
The idea behind 18+ art is mostly covered under "naughty" b/c gore and violence really doesn't pop up as much in our art even though we DO have a dark art category for awards.
Honestly, since there's NO rule about it gore/violence depicted in art, you COULD get away with it, BUT I WILL say that MA+ art would be
anything that could NOT be viewed by
everyone. If you felt that a 14 year old might have issues with it, or it might be TOO much for someone to see who might have lived through that, its best to go a head and mark it as MA+.
Material that is religious or questionable in general would also fall under this.
Mostly our issue is that unlike fanfiction with art there is E for everyone, and MA for everything else. You'd be so shocked at some of the lil stuff in some great works on here that have made that art work be marked as MA+ simply b/c of something tiny.
Thank you for asking though in advance!