Spolia" group on Flickr has grown to 39 members and 206 pictures since its inception on December 27 of last year. I don't anticipate maintaining such a rate of growth, though. I've searched most of the obvious tags (e.g. spolia, re-used, recycled) in as many of the languages as I felt I could do so profitably (English, Italian, French, Spanish, German). Future additions will be more dependent on self-submission by members than on invitations by me. Of course, there are many pictures out there showing spolia without saying so in their description, which makes it harder to find them.
The locations represented are concentrated in Mediterranean Europe, with a good showing from the British Isles, along with, thus far, one from China and one from Chicago. I've decided to keep Spolia broadly defined, including, for instance, the Lothar Cross with its Roman cameo of Augustus embedded in a 10th-century cross.
The Duomo in Pisa is particularly well-represented in the pool with 8 pictures, no doubt due to the frequency with which it is touristed. The Yerebatan Sarayı or Basilica Cistern in Istanbul is very well represented on flickr as a whole, but thus far only one picture has been added to the Spolia pool. Likewise, many pictures of a wall on Paros have been invited, but few have thus far accepted.
For purely epigraphic considerations, the best groups on Flickr that I've seen are "Visible Words - Visibile Parlare" for Latin and the Latin alphabet (1500 photos) and Greek (203 photos). There's also Inscriptiones, but it seems to have died with 19 photos.