14. The computer-implemented method according to claim 4 , wherein the at least one parent category in the portion of the categories comprises each parent category in the portion of the categories.
4. A computer-implemented method for searching a collection of content, comprising: under control of one or more computer systems configured with executable instructions, identifying a visually significant subset of categories of a category tree that categorizes the collection of content, the collection of content corresponding to a plurality of images, the visually significant subset including those categories of the category tree that are each associated with a respective subset of the plurality of images having a respective size within a specified range; receiving a search request associated with a query image; identifying, among the visually significant subset, a portion of the categories corresponding to the query image based at least in part on the search request, wherein at least one parent category is automatically excluded from the portion of the categories when the portion of the categories includes at least one child category of the at least one parent category; determining feature vectors for the query image and cluster descriptors for images of items associated with the portion of the categories; determining scores quantifying visual similarity between each of the images of items associated with the portion of the categories with respect to the query image based at least in part on the feature vectors of the query image and the cluster descriptors for the images of items associated with the portion of the categories, a respective score of each image of an item with respect to the query image being weighted based at least in part on a position of a category of the item in the category tree; selecting a set of the images of items associated with the portion of the categories based at least in part on the scores; and providing information corresponding to the set of the images for presentation.