In my academic paper, I have well over 100 figures, and would like to use an artwork I found online in one of my figures - it is not published in any book, it is just something they made and posted online. I would be using 1/6th of the author's artwork figure, and it would be so heavily modified it would barely be recognizable. The modified artwork would take up 1/40th the space of 1 such diagram (out of 100+) and used once only, it is a minor element of my figure.
(1) Do I need to cite the artwork in this case?
(2) Can you give any guideline on when an artwork citation is needed?
(3) Am I required to get permission from the author, or is it a courtesy to the author only?
(4) How would one cite an artwork in their dissertation?