Harmony in art is about creating a visually stimulating image by combining similar elements throughout the frame. Some elements that can be used are color, shape, and texture.
The above image uses harmony in a few different ways. The orange color brings together the whole image since it is spread out evenly throughout the entire image. Harmony without contrast, however, is boring. The orange is dark, light, blurred, sharp, big, small, and overall varied infinitely.
The harmony in this image comes from the green moss covering each of the rock. The contrast is added by the light and dark areas on each one.