The Index of Medieval Art Database will become perpetually ‘free to use’ from 1st July 2023 onward, for “researchers at all levels”. The largest database of such research, it is well-established and includes a “photographic archive” which offers iconographic clustering and links to referenced texts (e.g. Arthurian + Sir Lancelot pictures can be clustered together, and apparently there are also links to the story-texts related to each image). It also seems to includes carving, engraved items (spoons etc) and so on, and the definition of “art” appears to be as wide as you might expect or require for answering research questions (e.g. “plain as a pikestaff” — how plain and unadorned were medieval English ‘palmer’ pilgrimage staffs, exactly, compared to the staffs of officials and merchants?).

The Index of Medieval Art is currently public and free for initial searches, though “Subscription is required to view images” even for the thumbnails in the search results.