At some point during our visit to Saint Peter's Basilica I read that none of the paintings inside the church are actually paintings. They are in fact mosaics made up of extremely small tiles.
I'm sure it was Rick who said it, and he gave a reason that I can't remember. Unfortunately, the guidebooks have been returned to the library and my internet searches are not yielding the answer to the question why mosaics instead of paintings. Did it have something to do with damage? I'm sure it did.
In any case, it was hard to believe from a distance. All of the artwork appeared to be oil paintings to me. However, if you got really close, you could see the tiny mosaic tiles. Here is a close-up of the above mosaic.
I wish I could recall which particular martyr or Bible story this mosaic illustrated. Whatever the story, it's an amazing work of art.