Again, no canon for me...
Real
Photo 1:
A Subject that is a triangle itself -

Photo 2:
The subject make a triangle by perspective, converging toward the top of the frame

A hanged fishing net in the port of Acre, Isreal.
Photo 3:
The subject make a triangle by perspective, converging toward the bottom of the frame

Implied:
Photo 4:
Still-life arrangement of subject that produce a triangle with apex at the top

Photo 5:
Still-life arrangement of subject that produce a triangle with apex at the bottom

Photo 6:
3 people to make a triangle

My brother with his girls
I looked at the text book to find a tri-exmple for triangles. (I know, it's not funny!):
- "Willow trees in winter" by Albert Renger-Patzsch. A perspective triangle converging towards the top of the photo.
- "Plaquemines Parish, La. Oct.1935. An Unemployed Trapper" by Ben Shahn, Implied triangle creates from the lying figure, the legs and the arm on the left up to the head.
- "The mother and two children" by Carl Mydans, implied triangle made from three faces.