Anything related to digital image processing, i.e. the theory and the techniques used to extract or manipulate information from digital images.
Image Processing involves any kind of signal processing in which the input is an image. The output can also be an image, or it can be information relative to the input image.
Most image processing techniques involve treating the image as a two-dimensional array and applying standard signal processing techniques to it. Though optical and analog image processing are both well defined in the space of image processing, the tag image-processing in SO solely refers to digital image processing.
A good start on the field is having a look at the next links:
For an in-depth collection of explanations of a number of image processing topics, have a look at CVonline.
