Final answer:
Brightness determines the overall lightness or darkness of an entire image. While contrast refers to the variation between values, brightness has a direct impact on the image's perception along the light-dark spectrum.
Step-by-step explanation:
The aspect that determines the overall lightness or darkness of an entire image is brightness. Brightness affects whether an image appears more toward the white end of the spectrum (high-keyed) or the black end (low-keyed). The options contrast, color, and style influence the final project but in different ways. Contrast refers to the amount of variation between the highest and lowest values in a work, such as in photography, and can bring vitality or unity to the art piece. However, it is brightness that is directly responsible for the perception of lightness or darkness of the entire image.