1. A computer-implemented method, comprising: receiving, by one or more computing devices, a video; identifying, by the one or more computing devices, a key frame that is representative of subject matter depicted by the video; identifying, by the one or more computing devices and in a database, a stored image that has at least a specified level of visual similarity to the key frame based on visual features of the key frame and the stored image; determining, by the one or more computing devices and based on logging data corresponding to presentations of the identified stored image, that a keyword that triggers display of the identified stored image with various web pages provides at least a specified click through measure for the identified stored image when display of the identified stored image is triggered by the keyword; and outputting, over a communications network, based on the determination, and based on the visual similarity between the key frame and the identified stored image, data that suggest the keyword as a keyword that triggers display of the video.