For each image in a set of images, a representation of the image at a first level of quality is derived using a first representation of the image at a second, higher level of quality and is output for processing by a decoder. Configuration data is output for processing by the decoder to enable the decoder to detect whether or not the first representation of a given image in the set of images is to be reconstructed using residual data for the given image, the residual data: (i) being useable to reconstruct the first representation using a second representation of the image at the second level of quality, the second representation being based on the representation of the image at the first level of quality, and (ii) being derived based on the first representation and the second representation.