Triangles can be used in different ways to create a striking composition. In the image of the couple above their heads and shoulders have created a symmetrical triangle, which makes the image feel stable and harmonious – exactly the feel you want for a young couple in love.
Triangles are also a good way of linking three separate focal points within an image, rather than having three subjects along a line (which can also work too!)
Once you start looking for triangles in images you will see them everywhere!
Sometimes the triangle will be incomplete (with one of its apexes out of the image) but it still works as a way of making the image feel dynamic.