1. A method of comparing in a processor a logo image with a search image so as to detect the presence in the search image of a logo portrayed in the logo image, the method comprising: determining the positions of logo edges within a sequence of logo edges along a first spatial sampling direction in the logo image; representing said logo edges by ratios of distances between the respective logo edge and a plurality of preceding logo edges; determining the positions of search edges within a sequence of search edges along a first spatial sampling direction in the search image; representing said search edges by ratios of distances between the respective search edge and a plurality of preceding search edges; and comparing said represented logo edges with said represented search edges; wherein first and second logo image boundary positions are calculated from each matched sequence and respective frequency of occurrence measures are calculated for quantised values of first and second logo image boundary positions; wherein a logo image width is calculated from each matched sequence and widths are summed for each quantised value of first logo image boundary position and for each quantised value of second logo image boundary position; wherein a detected logo size and position is derived from an associated pair of first and second logo image boundary positions selected according to the difference between their respective quantised positions and their associated sums of width values; and wherein the presence in the search image of a logo portrayed in the logo image is detected when a weighted measure of the number of matches between sequences of edges at selected positions in the logo image and sequences of edges within a region in the search image bounded by detected logo image boundary positions exceeds a threshold.