1. A method performed by data processing apparatus, the method comprising: identifying a set of seed descriptors for an image in a document that is hosted on a website; generating, for each descriptor, structure information that specifies a structure of the document with respect to the image and the seed descriptor; generating, for each seed descriptor, one or more templates using the structure information for the seed descriptor, each template including: image location information specifying a location of the image within the document; document structure information specifying the structure of the document with respect to the image and the seed descriptor; image feature information specifying one or more feature values of the image, each feature specifying a visual characteristic of the image or data regarding an image file in which the image is stored; and a generative rule that generates descriptive text for other images in other documents; for each of the other images: generating descriptive text for the other image using the templates and the other document; and associating the descriptive text with the other image.