9. The system of claim 8 , wherein the instructions cause the image classification apparatus to perform operations further comprising: identifying, for a given user session, a first time of a user interaction with the reference image; identifying subsequent times, during the given user session, of user interactions with other images; and identifying, as co-selected images, only the other images having subsequent times of user interaction that are within a specified time of the first time, wherein the given user session has a session duration and the specified time is less than the session duration.
10. The system of claim 9 , wherein the instructions cause the image classification apparatus to perform operations further comprising: representing the reference image and co-selected images as nodes in a weighted graph, each node being connected to other nodes by weighted edges representing sequential selections of the images represented by the node and the other nodes; and wherein the measure of relevance for each co-selected image is generated based on the weighted edges.