Formal elements are what decides the composition and aesthetic of an image in photography. Such examples include: (My own photos)
Lines: Could be of any direction; straight, diagonal, vertical, horizontal, etc. It helps guide the viewer’s eyes into a sense of direction.
Shapes: Could be geometric or organic and create visual interest to a specific photograph.
Pattern: Could be a repetition of a certain subject, creating interesting visuals to present to the audience.
Texture: The surface quality of objects, could be smooth, rough. Adds visual interest and imagery to the photo.
Tone: Refers to the lighting or darkness of the photograph, and helps create visual contrast and creates interest on the photo.
Focus: The sharpness/clarity in a photo that helps create emphasis on the subject in the photo to create visual interest.
A visual appealing image includes multiple elements as photos will inevitably include lighting, shape and focus. However, it can focus on one specific element in the photo to create specific visual interest for that element. For example, in the texture photo, it emphasizes the roughness of the tree bark. However, focus is also included: it perfectly focuses on the tree and includes bits of colour.