1. A method performed by data processing apparatus, the method comprising: for each of one or more textual advertisements of a sponsor of the textual advertisements, each of the advertisements including a link to a corresponding landing page that causes a user device to request the landing page in response to the advertisement being selected when the advertisement is displayed on the user device: identifying landing page images in the landing page to which the textual advertisement links; for each landing page image identified in the landing page, determining a relevance measure that measures the relevance of the landing page image to the content of the landing page; selecting, by a data processing apparatus, one or more of the landing page images for concurrent display with the textual advertisement based on the relevance measures of the landing page images; and storing, in a data storage system, data associating the selected landing page images with the textual advertisements.