What image is ?

Images may be two-dimensional, such as a photograph, screen display, and as well as a three-dimensional, such as a statue or hologram. They may be captured by optical devices–such as cameras, mirrors, lenses, telescopes, microscopes, etc. and natural objects and phenomena, such as the human eye or water surfaces.
The two-dimensional (2-D) discrete, digital image I (m, n) represents the response of some sensor or camera at a series of fixed positions (m 1,2,…….M; n 1,2…….N) in 2-D Cartesian coordinates and is derived from the 2-D continuous signal I (x, y) through a sampling process frequently referred to as discretization. Discretization occurs naturally with certain types of imaging sensor (such as CCD cameras) and basically effects a local averaging of the continuous signal over some small (typically square) region in the receiving domain.
The indices m and n respectively designate the rows and columns of the image. The individual picture elements or pixels of the image are thus referred to by their 2-D (m, n) index.

Following image will demonstrate the image Cartesian image coordinate system :