Thanks to a reader who sent me the following information. The database of the 'Index of Medieval Art' of Princeton University, which is normally accessible in just a few research libraries because of the high subscription costs, is, due to coronavirus, freely available on the internet until June 1st (https://ima.princeton.edu/). 

 This Index started as a library of early Christian imagery (hence the original name 'Index of Christian Art'), but it was later expanded to include images of any classification until 1400. It is primarily a database for scholarly research, with often small images (many are still in b/w, but there are also decent color images of most, if not all, manuscripts in the Pierpont Morgan Library) and very rich descriptions, that allows users to quickly find images of specific iconographies. 

If one follows the 'Browse' section (see screenshot) one can find long lists of illustrations of liturgical scenes. 

One would naturally hope that this resource might remain public after the current crisis, so it may not harm to write to the Index praising its importance even outside the academic community.

There are photographs not just of painting, but of art on liturgical artifacts too. For example, you can follow this link to see a gothic image of the personification of prudence on a crozier. Copyright issues do not allow me to show the photograph directly.