In the case of paintings, we just list the painter as all three art roles (cf. Alex Ross). Even inker, though you could probably also keep that blank. Digital artwork would probably be the same. The issue could be mentioned in the Notes section.
Listing a painter as an "inker" is a bad idea. Painters are painters, not pencillers. And, again, there are other media than painting--it's not sensible and outrite misleading to mark them as something they aren't.