A number of museum websites offer downloadable high-quality images for academic purposes in conjunction with thorough metadata. Sites with particular strengths in fifteenth- and sixteenth-century ceramics are:
British Museum: http://www.britishmuseum.org/research.aspx
Victoria & Albert Museum: http://collections.vam.ac.uk/
Metropolitan Museum: http://metmuseum.org/collection/the-collection-online
Getty Museum: http://www.getty.edu/art/
The challenge for the researcher is that each database is different, thus making it difficult to archive images in a seamless way.
The Museo Correr in Venice also has an important collection. Much of it is now available online with color images and an improved search system (in Italian): http://correr.visitmuve.it/it/catalogo-opere/