Instead of the black and white marble tiles that I had planned originally, I now have contrasting Moroccan tiles on the courtyard! The muted colors of the tiles go nicely with the river-rock foundation around the courtyard.
I used Photoshop to resize some images of tiles that I liked. Although the tiles look gray, they have a slight sepia tone. I then decoupaged the tiles over the existing paperclay layer. Instead of adding a glossy varnish, I liberally smeared wood-glue on the tiles, giving a matte aged finish which photographs nicely.
The tile surface is uneven to the touch, since the paperclay layer below was textured. But I think the slight sheen and imperfections add to the old tile feel.
If you click on the image below to magnify it, right click and "View Image" you should be able to download and print the high resolution version of the tiles for your own projects.