The image is the "Vita di Cristo" or Christ's Passion, a fresco on the partition ("Parete Gaudenziana") of the Santa Maria delle Grazie Church in Varallo Sesia, Italy, painted by Gaudenzio Ferrari in 1513. It depicts the life and death of Christ in 21 panels, with the crucifixion prominently displayed in the center.
This is probably the largest digital image on the web: A total of 1145 frames were taken by digital camera and assembled into a final gigantic 8.6 gigapixel by HAL9000 S.r.l. (no, not that HAL 9000)
You can zoom in to your heart's content, up to a close-up distance of 1.71 cm (a little more than half an inch)!
http://haltadefinizione.deagostini.it/ [Flash] - via Very Spatial
The interface for playing with it is quite clever and nice, too.
[rubs hands together] NOW I can get started on MY OWN DaVinci Code type project!!! Bwahahahahaahaaa!