1. A computer-implemented method, comprising: obtaining a reference label for a reference image, the reference label being a label that specifies a first topic to which the reference image belongs; identifying, in a data processing system, user sessions during which the reference image was selected for presentation, each user session being a period for which selections of images are associated into a session set; identifying, in the data processing system, co-selected images for the reference image that were selected for presentation during the user sessions, a co-selected image being an image that is selected subsequent to selection of the reference image; for each co-selected image, generating, in the data processing system, a topic score based on a frequency of selection of the co-selected image for the reference image, the topic score representing a measure of relevance of the co-selected image to the first topic; and labeling co-selected images having topic scores that satisfy a threshold topic score as belonging to the first topic.