Triangles in art can give various meanings.
They can give the apearance of stabillity and strength in faces, for example. Cezanne used triangles to compose faces, using features like the eyes and cheek bones. Picasso used triangles to make solid portions of things. Michelangelo used triangles to compose his sculptures in pyramid form, and conveyed strength (figurative and structural) through it.
Triangles are also good for directing the eye through a piece. Tall, lengthy triangles drag the eye toward their tip. For example, in a long photo of trees, the trees make upward arrows. Columns are built to taper down as they get taller, forcing the eyes up. In churches, the spires are pyramid-shaped to move the eyes toward the heavens.
Today, triangles bring to mind technology and the future with their clean edges and geometric patterns, but triangles have been used since antiquity.