Do I need to cite an artwork? How would I do it? Is permission needed?
Simply contact artwork's author (since the artwork wasn't published, the author is copyright holder, otherwise it usually would be the publisher), requesting permission to adapt the artwork. I suggest sending the author planned version, so that any potential issues can be assessed and addressed.

(1) I'd say Yes. The citation is usually located below or above the figure in the following form:


  (Adapted from “The DeLone and McLean model of information systems success: A ten-year update.”, by DeLone and McLean, 2003, Journal of
  Management Information Systems, 19(4), p. 24. Copyright 2003 by M.E.
  Sharpe, Inc. Adapted with permission.).


(2) Since artwork is someone's intellectual property artifact, protected by copyright, it is usually (for an exception, see fair use in the next point) required to cite the artwork (as any other such artifact) to properly credit its authors. The format of citation depends on the publication style you're using (my example above is based on APA Style).

(3) You're required to get permission, unless you want to risk by using the fair use legal doctrine.

(4) If the artwork is part of a figure, see example above; otherwise, other relevant standard citation guidelines for visual elements apply, based on the publication style you use.