Here is a list of some of the major online sites which either contain some collection of Medieval art or imagery or the site is a major resource for art research.
British Library site has access to many fantastic images and resources including the catalog of illuminated manuscripts which allows text search-ability and full color images of many Medieval manuscripts.
This online resource has digitized copies of New York Public Library's collection of Medieval manuscripts which is one of the larger ones in the United States.
Their site has a wealth of images of Medieval Art and Manuscripts from their collection. It has one of the more well known collections in the United States.
