Use explicit for lewd or guro, questionable for ecchi and safe for everything else.
Seriously don't forget it. It's annoying to find SFW pics when I'm looking for porn.

Hold up, don't ecchi and lewd mean the same thing?

Explicit - Sex, visible sex organs, guro
Questionable - Panty shots, groping, suggestive poses (lewd)
Safe - Everything else