1. A computer-implemented method comprising: comparing image data that specify optical content depicted by a first advertisement to image data that specify optical content depicted by a reference advertisement, wherein the reference advertisement is associated with at least one keyword; determining, by one or more processors, a similarity measure between the first advertisement and the reference advertisement based on the comparison of the image data; associating one or more of the keywords from the reference advertisement with the first advertisement based on the similarity measure between the first advertisement and the reference advertisement; receiving a request for an advertisement, wherein the request includes a keyword; and responsive to the request, delivering the first advertisement in response to the keyword of the request matching a keyword of the one or more keywords associated with the first advertisement.
2. The method of claim 1 , wherein associating one or more of the keywords from the reference advertisement with the first advertisement comprises: determining that the similarity measure between the first advertisement and the reference advertisement exceeds a similarity threshold.