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.