As someone who has spent a few hundred hours quote-wrangling, I get a little tired of the more popular quotes by famous artists. Here a couple of personal faves of mine:

Art, like morality, consists in drawing the line somewhere.
G.K. Chesterton

When my daughter was about seven years old, she asked me one day what I did at work.  I told her I worked at the college – that my job was to teach people how to draw.  She stared at me, incredulous, and said, “You mean they forget?”
-Howard Ikemoto