Residual data is obtained based on first and second representations of an image at a first level of quality. The second representation is based on a representation of the image at a second, lower level of quality. The residual data is useable by a decoder to reconstruct the first representation using the second representation. A configuration message comprising a header part and a payload part is generated and output for processing by the decoder. The payload part comprises obtained configuration data relating to processing of the residual data. The header part comprises: (i) a message type parameter specifying a configuration message type and indicating a given payload format of the payload part, a format size of a payload part having said payload format is predetermined or can be determined from the content of the payload part, and (ii) a payload size parameter specifying the actual size of the payload part.