1. A method performed by data processing apparatus, the method comprising: identifying a set of one or more seed descriptors for a given image in a given document that is hosted on a website, wherein each seed descriptor has been identified as being a correct description of the given image; for each seed descriptor: identifying a location of at least one word of the seed descriptor in the given document by comparing each word of the seed descriptor to text included in the given document; in response to identifying the location of the at least one word of the seed descriptor in the given document, generating, based on the identified location of the at least one word of the seed descriptor in the given document, structure information that specifies a structure of the given document with respect to the given image and the seed descriptor, the structure specifying a location of a string of text that includes the at least one word of the seed descriptor within the given document with respect to a location of the given image within the given document; and generating one or more templates using the structure information for the seed descriptor, each template including: image location information specifying a location of the given image within the given document; document structure information specifying the structure of the given document with respect to the given image and the seed descriptor, including the location of the string of text that includes the at least one word of the seed descriptor within the given document with respect to the location of the given image within the given document; image feature information specifying one or more feature values of the given image, each feature value representing a respective visual characteristic of the given image or data regarding an image file in which the given image is stored; and a generative rule that generates descriptive text for other images in other documents; and for each generated template: identifying a set of documents that have (i) document structure information that matches the document structure information specified by the generated template and (ii) an image that has image feature information that matches the image feature information of the given image; and for each document in the set of documents: generating descriptive text for the image of the document using the generative rule of the generated template and the document; and associating the descriptive text with the image.