1. A computer-implemented method of annotating images in a data set comprising a first set of images which initially have annotations and a second set of images which initially do not have annotations, the method comprising: determining spatial correspondences for all pairs of images in the data set, wherein each spatial correspondence represents a mapping of a location in a first image to a corresponding location in a second image, including determining a first mapping of a first location in the first image to a first location in the second image and a second mapping of a second location in the first image to a second location in the second image, wherein the first location in the first image is different from the second location in the first image, and the first location in the second image is different from the second location in the second image; for each mapping, calculating a connection strength between the location in the first image and the corresponding location in the second image, wherein the connection strength is determined using a pairwise measure of similarity between one or more features in a pair of images that is calculated locally on a restricted portion of the pair of images, such that the connection strength is spatially variant across a pair of images, including calculating a first connection strength for said first mapping and a second connection strength for the second mapping, wherein the first connection strength for the first mapping is different from the second connection strength for the second mapping; and iteratively propagating information across images in the data set using: (i) the connection strengths of the mappings, including the first and second connection strengths of the first and second mappings respectively, and (ii) an estimate of the accuracy of the information which is being propagated, as weights for propagating the information, wherein the propagated information is used to provide annotations to images which do not initially have annotations, and wherein each iteration performs a propagation across all images in the data set.