1. A method performed by data processing apparatus, the method comprising: obtaining a group of similar images for a particular image, wherein each similar image is associated with one or more n-grams; determining an overall score for each of the n-grams, wherein for each n-gram the overall score is based on a combination of image specific scores for the n-gram; and selecting one of the n-grams as a textual description of the particular image according at least in part on the overall scores for the n-grams and the respective order of each of the n-grams.