Look above the doors on older streets in Lisbon and you will sometimes find a small panel of tiles with a saint on it. These are registos. After the earthquake of 1755, as the city was rebuilt, small devotional panels like these began appearing on buildings as protection against future disasters. Some were placed there to ask for protection over the building and the people who lived in it.
Inside the grand houses, tile did a different job. In the eighteenth and nineteenth centuries, palace entrances were given figuras de convite: life-size figures cut from azulejo panels, footmen and elegantly dressed ladies standing at the door to greet whoever arrived. At the Palace of the Marquises of Fronteira, the tiles include monkeys playing trumpets. The tradition has never been too serious to make room for an animal.
When São Bento station was built in Porto, Jorge Colaço spent eleven years, from 1905 to 1916, covering its walls with around twenty thousand tiles. Beside the battles and the kings, he put the vineyards, the harvest and the wine going down the Douro. Ordinary people at work, on the same walls as the kings.
So a tile with your grandmother's face on it, or your dog's, fits a long Portuguese habit: putting the people and places you care about on the wall, where everyone who comes to the door can see them.