1. An annotation system comprising: memory which stores: a structured prediction model comprising a graphical structure which represents predicted correlations between values assumed by labels in a set of labels, the graphical structure comprising at least one tree structure, wherein in at least one of the tree structures, each of the labels in the set of labels is in exactly one node of the tree structure, the nodes of the tree having at most a predefined number k of the labels, and each of a plurality of the nodes has more than one of the labels, and edges between the nodes define those pairs of nodes for which predicted correlations between the values of pairs of their labels is used in the label prediction; memory which stores instructions for: generating feature-based predictions for values of labels in the set of labels based on features extracted from an image; and predicting a value for at least one label from the set of labels for the image based on the feature-based label predictions, and the structured prediction model, and, when the instructions include instructions for receiving an assigned value for at least one label from the set of labels for the image, the predicted value being also based on an assigned value for at least one other label, if one has been assigned; and a processor for executing the instructions.