That's the rub... there's nothing to suggest the image(s) aren't open domain content. They're presuming guilt... the opposite of the burden of proof that exists here in the States.
It's been in their discover and trending sections repeatedly and people are complaining.... Webtoons is AWARE of it. This isn't something that's slipping by their attention. Maybe they're still doing their due diligence but the inaction, to this point, lends itself to acceptance.
Yup ...but they would have a responsibility to remove it if they were made aware of it, and the claims were true. We're not seeing that happening so either they deemed the potential complaints false or have been unable to verify said claims.