1. A decoder for decoding a signal including digital data representing an image of either a first or second image resolution, the decoder comprising: means for storing at least a portion of the digital data representing the image; means for detecting whether the image is of the first or second image resolution, the second image resolution being higher than the first image resolution and using more data to represent the image than the first image resolution; means for processing the digital data representing the image to reduce the amount of digital data used to represent the image, when it is detected that the image is of the second image resolution, thereby generating a reduced set of image data; and reduced image data storing means for storing the reduced set of image data generated by the data processing means.