17. A system, comprising: one or more processors; and a computer-readable medium coupled to the one or more processors having instructions stored thereon which, when executed by the one or more processors, cause the one or more processors to perform operations 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 the keyword of the request matching a keyword of the one or more keywords associated with the first advertisement.
17. A system, comprising: one or more processors; and a computer-readable medium coupled to the one or more processors having instructions stored thereon which, when executed by the one or more processors, cause the one or more processors to perform operations 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 the keyword of the request matching a keyword of the one or more keywords associated with the first advertisement. 21. The system of claim 17 , wherein determining a similarity measure between the first advertisement and the reference advertisement comprises: identifying a link included in the first advertisement and a link included in the reference advertisement; and determining the similarity measure based, at least in part, on whether the link included in the first advertisement matches the link included in the reference advertisement.