1. A computer-implemented method comprising: obtaining a query image that has been submitted by a user of an image search system; determining, by the image search system, that the query image is indicated to be associated with an object; obtaining, for the query image: (i) one or more training images of a corpus of training images that are indicated to be associated with the object, and (ii) for each one of the one or more training images that are indicated to be associated with the object, a similarity score that reflects a level of similarity between the query image and the respective training image; determining to add the query image to the corpus of training images that are indicated to be associated with the object in response to the similarity score that reflects the level of similarity between the query image and the respective training image satisfying a threshold; and training the image search system to recognize the object in subsequently received query images, using the corpus of training images that are indicated to be associated with the object.