My first guess was that they printed onto transferable tattoo paper, but this would mean silkscreen, not digital. My new first guess is that this was printed using the gliclee imaging process, which uses water based inks, like an ink jet printer, and then the image is placed on the wet plaster. If this was the technique used, it would be quite expensive but very high quality and capable of large sizes.

There was an Italian company, which I lost track of, thatwas selling "fresco" using water pigments on canvas which you transferred to your wet wall.

There is another Italian company selling some sort of photos, actually mounted in plaster panels, but they do not look very nice (from internet photos). Another Florida company www.antiquitiescollection.com has yet another "fresco" printing process. Of interest at this site is the concept of mounted tile frescoes, true fresco, it would be possible to mount plaster panels in easy to manage sections (with metal mesh, about 7 lbs per sq ft. each one carefully numbered), intonaco then paint. Disassemble after hardening, and ship for reassembly on site. The first reaction will be "what about the seam lines?", but the antiquities site shows that this is not too much of a problem.

Anything it takes to kickstart the market!