The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued.  Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. 
Image retrieval systems allow users to use a client to retrieve a set of digital images that match a set of search criteria.  For example, many websites allow a user to submit one or more keywords to a server.  The keywords are processed by the server to determine a set of images that are associated with the submitted keywords.  The server may then display the matching set of images, to the user, on a subsequent webpage. 
Certain image retrieval systems may also allow a user to select a digital image.  The user may then cause the image retrieval system to retrieve a set of digital images with a visual appearance similar to that of the selected digital image.  For example, a user may view a webpage with a several digital images displayed thereon.  The user may select a digital image of a giraffe on the webpage.  In response, the server may retrieve a set of digital images whose visual appearance is similar to that of the selected digital image of the giraffe, and thereafter display those similar looking digital images on a subsequent webpage. 
Unfortunately, current techniques in the art for retrieving a set of digital images that match a set of search criteria are not very accurate.  Thus, approaches for improving the accuracy in retrieving a set of digital images that match a set of search criteria are desirable.