They can be still-life or the focus of the scene which includes people. Raw or cooked. Just make them interesting.
Things that are old, discarded, meaningless or cheap can make an interesting image. Junkyards, dumps, and other things often have junk.
Let your camera look up or down. Upward can make the subject seem grand, and downward can make it look vulnerable or insignificant.
Which color doesn’t matter, but there can only be shades of one in the image. Examples are black and white (that’s two but they are not really colors), and sepia. But any color will work.
Man-made things. Examples are buildings, bridges, docks.
An image that is taken in the confines of a built-up area. Can be architecture, the streets or the interaction between people and their environment.
Contrast is determined by the difference in the color and brightness of the object and other objects in the same field of view.
Use reflective surfaces to create and artistic echo of a scene. Examples of reflective surfaces are mirrors, oceans, lakes, puddles and even rain drops.
A person or persons. Make it special. Examples are portraits, photojournalism, landscapes and street photography including a person or persons.
Make something create the impression of movement, either of the subject or the camera.