If an aesthetically pleasing design is important to selling a product, it stands to reason any face drawn in an ad for your product should also be aesthetically pleasing. This is best done by using the golden ratio.
“This number, universally recognized as ideal, is usually rounded to 1.62. It is also known as phi (Φ) in Geometry, Fibonacci numbers in India, feng shui in Asia, and divine proportion. This ratio 1:1.62 is nature’s symmetry and occurs naturally in life. Da Vinci himself used it when he drew the perfect human male body in his famous work the Virtruvian Man. The premise behind this is that the closer a face or object is to the golden number 1.62, the more beautiful it becomes.”
Click Here to gauge if your avatar is proportioned to the golden ration. This page was recommended to me by a dermatologist who uses these principals in cosmetic procedures.