1. A method of generating labeled images of objects belonging to a particular object category, the method comprising: obtaining data identifying the particular object category; identifying database images of the objects belonging to the particular object category, comprising: generating a search query derived from a label for the particular object category; selecting a plurality of candidate videos from videos identified in response to submission of the search query; selecting one or more initial frames from each of the candidate videos; detecting one or more initial images of objects belonging to the particular object category in the one or more initial frames; for each initial frame including an initial image of an object belonging to the particular object category, tracking the object through surrounding frames to identify one or more additional images of the object; and selecting one or more images from the one or more initial images and one or more additional images as the database images of objects belonging to the particular object category; and storing the database images as images of objects belong to the particular object category.