An essential site for information about, and images of, anything related to ancient Greece is the Perseus Digital Library. Wonderful. Check out the very useful search facility. (If it's down, there are mirror sites in Oxford and Berlin.)
Steve Esposito of Boston University has a number of online materials to complement his 1998 translation of the play. These include outlines of key speeches, and a few quotations about the uniqueness of the Bacchae and about the uniqueness of Dionysus.
A study guide by Roger Dunkle; a snazzier-looking version of the same guide.
Another study guide, this one by Robin Mitchell-Boyask.
Links to images of Dionysus.
