19. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising: receiving, for each image in a set of images, a corresponding set of labels, each label being determined to be indicative of subject matter of the image; for each label, determining one or more confidence values, each confidence value being a measure of confidence that the label accurately describes the subject matter of a threshold number of respective images to which it corresponds; identifying labels for which each of the one or more confidence values meets a respective confidence threshold as high confidence labels; for each image in the set of images, identifying the labels in its corresponding set of labels that are high confidence labels; and identifying images having a corresponding set of labels that include at least a respective threshold number of high confidence labels as high confidence images.
19. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising: receiving, for each image in a set of images, a corresponding set of labels, each label being determined to be indicative of subject matter of the image; for each label, determining one or more confidence values, each confidence value being a measure of confidence that the label accurately describes the subject matter of a threshold number of respective images to which it corresponds; identifying labels for which each of the one or more confidence values meets a respective confidence threshold as high confidence labels; for each image in the set of images, identifying the labels in its corresponding set of labels that are high confidence labels; and identifying images having a corresponding set of labels that include at least a respective threshold number of high confidence labels as high confidence images. 26. The non-transitory computer storage medium of claim 19 , wherein the instructions upon execution cause the data processing apparatus to perform further operations comprising providing the high confidence images to an image annotator as training images.