Shrines and crosses you find on the road count for this, obviously. But so do artworks in churches. Anything that says "read" in the church counts. The church in Sausa (not monastery), the two churches in Rattay, and both Rattay castles have artwork.
You need 90 total to get this. It's tracked in the stats page under Art Objects Found I believe it is, but Art is definitely in the name.
You must click read on the object for it to count, be it cross, shrine or art.