This paper give a brief survey of the technologies for image retrieval based on features of the image content and also several online systems providing online image retrieval service implemented with these technologies. Because there are the larger amount of image distributed on the Internet than before, it is very important to know some online systems that provide this kind of service and to understand characteristics of the systems through analyses of related technologies for the users who have need of acquiring image information from the Internet. In this paper, we discussed various related technologies according to the improtant functional components usually constituted an image retrieval system, which include database management subsystem, feature extraction subsystem, input interface, feature matching subsystem, and output interface. We also recommend three different online image retrieval systems, xcavator.net, retrievr, and Piximilar, and a online shopping system based on the technologies, Like.com, for those readers who are interested to this subject matter.