1. A method of image transmission comprising repetitively: (a) generating an estimate of a picture area by interpolation from selected actual samples thereof to provide estimated samples; (b) comparing the estimated samples with the actual samples; and (c) if the estimated and actual samples differ by less than a threshold criterion, transmitting picture data representing the said actual samples, and if they do not, notionally dividing the area under consideration into two or more sub-areas and subjecting each sub-area to steps (a), (b) and (c) until a minimum sub-area size is reached.