For images of gods and goddesses, see Laurel Bowman's Classical Myth: The Ancient Sources.

The Hellenic Ministry of Culture maintains a web site with images from many museums and archaeological sites, including Eleusis and the Eleusis Museum.
 

There are some images of the site in Dr. J.'s Illustrated Eleusis.
 

An invaluable source for texts, images, and historical analysis of the ancient Greek world is Perseus.   For images, search for “Eleusis”, "Demeter", Persephone", or "Hades" in the search box at the top right corner of the top page, then browse the various links which will appear.
 

For a brief description of the Eleusinian Mysteries, see Thomas Martin's Overview of Greek History at the Perseus web site.
 

For more detail on the myths--much more detail--go to Greek Mythology Link and scroll down to the table of contents.  For convenience, here are direct links to Demeter, Persephone, and Eleusis.
 

A good account of the Mysteries and their possible significance, with full scholarly bibliography, is provided by the Ecole Initiative of Edward Beach.
 

An extensive discussion of the Mysteries can be found at The Mysteries of Eleusis, a web site prepared by Sanderson Beck.  Beck delves into the evidence provided by Christian writers, and has much to say about the deeper spiritual significance of the rites.  Speculative, but interesting.
 

For a wealth of information about women and gender in the ancient world, including translations, images, essays, and bibliographies, go to Diotima.