1. A method performed by data processing apparatus, the method comprising: receiving an image as a query from a user device associated with a user; deriving a textual query corresponding to the received image, wherein deriving the textual query for the image includes: using the image to determine a group of images that are similar to the received image; for each similar image of the group of similar images, identifying one or more n-grams associated with the similar image; and selecting one or more of the identified n-grams as a textual query for the received image; obtaining search results responsive to the textual query; and providing for presentation on the user device one or more of the obtained search results.