1. A method of representing an image comprising using a processor to perform the steps of: deriving at least one descriptor based on colour information and colour interrelation information for at least one region of the image, the descriptor being derived using values of pixels in said region and having at least two descriptor elements that are functions of values of pixels in said region, wherein at least one of said descriptor elements for said region is derived using only a subset of pixels in said region, wherein deriving said at least one descriptor includes performing the steps of: identifying said region of the image; determining the values of pixels in said region; performing a first calculation on the pixel values of said region to determine a first value that represents colour information of said region; performing a second different calculation on the pixel values of said region to determine a second value that represents colour interrelation information of said region, wherein at least one of said first and second calculations involves examining only a subset of pixels in the region; and forming a descriptor containing at least said first and second values as said at least two descriptor elements.