How should I use 'right' and 'left' when describing a person?

Is there a standard convention for using right and left when describing a person? Should it always be from the perspective of the person being described?

For example, when describing a flat object:

There was a ketchup stain on the right side of the painting.

This sentence is using "right side" from the perspective of someone looking at the painting.

For non-flat objects:

There was a bright, hot-pink streak on the right side of the car.

This is strange to me. It could go either way, since the car has a "front" and "back", it also has a "left" and "right", so the hot-pink streak could be on the right side from the car's perspective, or the right side from the viewers perspective.

The same applies to people:

He was an older man with a huge, curved scar dominating the right side of his face.

Does this mean that, from his perspective, the scar is on his right side, or his left side?

If you were facing someone and asked them to extend their left hand (i.e the one to your right), would you expect them to reach out with the hand to your left? If not, you have answered your own question.

But things are different with inanimate objects: with the painting, references to viewpoint (and hence any mention of handedness) relate to the observer. Though it should be noted that if you are describing an attribute of an animate object depicted in a painting, its own viewpoint will still supersede yours when it comes to distinguishing left from right.

As far as a car is concerned, the handedness convention assumes the viewpoint of someone who is facing forward in the car (i.e. in the direction of normal travel).

There are two specific, dedicated terms of art specifically introduced to resolve this ambiguity:

Proper right

and

Proper left

They mean "right" and "left" as the figure in the painting would see them.

_{image source: wikiart.com}

Thus, the Mona Lisa's proper right hand is folded over her proper left arm, because that's how she would describe herself, if she could talk.

For the sake of an authoritative reference, here is an excerpt from page 34 of Save Outdoor Sculpture! Volunteer Handbook:

For figurative works, use the abbreviations PR (proper right) and PL (proper left) to indicate the right or left side of the statue from the perspective of the statue (i.e., your right or left side if you were positioned on the base facing in the same direction as the statue).

The pamphlet is produced by "Save Outdoor Sculpture!" ("SOS!"), a joint project of the National Museum of American Art, Smithsonian Institution, and the National Institute for the Conservation of Cultural Property.

Stage left. Stage right. On your left. On your right. As viewed from the front. Left and right, other than dates, are the most perplexing. The navies of the world solved it with starboard and port. I like north-south, east-west when I can use it. There's also passenger side. The art world, I see from the above, solved it with "proper".