An image is a 2 dimensional representation of a scene. The camera sensor creates a two dimensional signal of a scene. This signal is sampled in space to get an array of dots called as pixels. Each dot represents a portion of the image in either black and white (or) color. Originally the dot is an analog voltage signal and this is quantized into a finite set of voltage levels. If ‘B’ is the number of bits that are used to represent the voltage levels, then the given total voltage range of the image signal can be divided into ‘2B‘ ranges. Any pixel value will then be mapped into one of these ‘2B‘ ranges through binary representation with ‘B’ bits.
The higher the number of pixels for a given area of image, the better is the quality as the pixel is representing a small portion of the original image and the gap between the pixels is lesser. In the same manner, if there are more number of bits ‘B’ used to represent each pixel’s analog voltage value, the better is the quality as the range in a given quantization level is lesser.
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References
Chapter 1 of Bovik, A. C. (2010). Handbook of image and video processing. Academic press.
Dr. Selvaraj Vadivelu 