Its also a courtesy thing. One above ain’t traced but it is obviously heavily referenced, and even references should be credited.  It is only fair to the artist who originally did the work.  I’ve had people eyeball work but put a different spin on it and still ask for permission or just credit anyways.  Not every artist will explicitly state their permissions for a piece or work so in general its a good idea to at least ask.

        Plagiarism: an act or instance of using or closely imitating the language and thoughts of another author without authorization and the representation of that author’s work as one’s own, as by not crediting the original author. (Sound familiar?)
       
    Sourcing isn’t just for term papers. Referencing isn’t evil or bad, but not crediting is. Giving credit is just etiquette everyone should follow, not just writers or artists.