1. A method for retrieving digital images, comprising: generating a content index comprising a plurality of entries, wherein each of the plurality of entries contains image description data that reflects one or more content attributes of one of a plurality of images; receiving, from a client, request data that requests one or more requested images that are each (a) associated with one or more keywords, and (b) similar to a base image; determining one or more potential images that are associated with said one or more keywords; analyzing the base image to determine content data, wherein the content data reflects one or more content attributes of the base image; determining the one or more requested images by: for each image in at least a subset of the one or more potential images, comparing the image description data, associated with an entry of the content index for that image, with the content data, thereby determining how similar the content of each image is to the content of the base image, and selecting a portion of the subset of the one or more potential images as the one or more requested images; and transmitting, to the client, response data that identifies the one or more requested images.