The invention has a sending device and a decoding device, where the sending device sends via a transmission channel to the decoding device at least one key image during a first time interval and a secondary image calculated using the at least one key image during a second time interval. The at least one key image is defined at least partly by base data and the secondary image is defined by secondary data. The sending device sends the base data to the decoding device during the first time interval, determines enhancement data for the at least one key image according to an available bandwidth of the transmission channel and the amount of secondary data to be transmitted, and sends the secondary data and the enhancement data for the at least one key image to the decoding device during the second time interval.