Image processing is defined as the task of processing a 2-dimensional array of numerical data to achieve some predefined goal. The goal may be as simple as to make the image look better (e.g. sharper) which is a common task in photographic post processing.
Input image -> Some operation -> Output image
The result of a typical image processing operation results in an output image which is of same data type as the input image and the same two axes of spatial coordinates.
The Bible: Digital Image. Processing. by Rafael C. Gonzalez. & Richard E. Woods