1. A computer-implemented method, comprising: obtaining a reference text label that specifies a first topic to which a reference image is relevant; identifying, in a data processing system and as a co-selected image for the reference image, an image that was presented in image search results and selected by a user during a same user session in which the reference image was presented in the image search results and selected by the user; generating, in the data processing system, a topic score for the co-selected image based on a number of user sessions in which image search results included both the co-selected image and the reference image and in which a user interacted with both of the reference image and the co-selected image, the topic score representing a measure of relevance of the co-selected image to the first topic; and labeling the co-selected image as an image that is relevant to the first topic based on the topic score meeting a threshold topic score.