1. A method of interpolating in a processor a new image at an output position between first and second existing images within an existing sequence of related images, comprising the steps of: determining motion vectors associated with pixels or regions of pixels of the first existing image in the sequence; shifting pixels of the existing image to the output position in accordance with the or each motion vector associated therewith; determining at least two probability of accuracy measures for each of a plurality of the said motion vectors: a first measure describing the probability that the respective associated pixel is newly revealed in the said first image of the sequence; and, a second measure describing the probability that the respective associated pixel is about to be concealed in the said first image of the sequence; forming a weighted sum of the said shifted pixels where the weight for each pixel is proportional to a combination of the probability of accuracy measures for its respective associated motion vectors; and creating said new image by addition of the said weighted sum of shifted pixels to a second sum of weighted shifted pixels derived from the second existing image in the sequence.