1. A method of determining an indicator for a presence of an image, said image notionally being made up of a two dimensional array of pixels, said method comprising the steps of: (a) imposing a grid of cells over a portion of the image; (b) determining a representative value for each of said cells; (c) determining a measure of the difference between representative values of adjacent cells of said grid, said measure being determined over the whole of said grid; (d) repeating steps (a) to (c) for substantially all possible positions of said grid of cells to determine an optimal grid position in which at least an adjacent pair of cells returns a maximum difference measure; (e) for each cell within said grid corresponding to said optimal grid position in which at least an adjacent pair of said cells returns a maximum difference measure, determining a range of representative values for said cells, said range being derived from the maximum and minimum representative values for said cell when said grid is subjected to a predetermined amount of rotation, scaling and translation or any combination thereof; and (f) utilizing said grid optimal position and said range of representative values for each cell within said grid corresponding to said grid position as an indicator of said image.