University of Cambridge provide access to a set of Medieval images, and provide commentaries on them. Basic search is - very basic.
You can choose in advanced search:
1. Media: type of painting used oil, tempura, gold and medium such as paper book.
2. Cycle: don't know what this is.
3. date: 11th to 16th Century
4: Title: of painting, any word or exact phrase.
5. Keyword: search restricted set of keywords e.g. Christ in Majesty, Evangelists, Majesty
6. Image number (if you know it).
Doesn't seem to be a full text search on the commentaries unfortunately.