Conversely, where is the actual Lena reference image? 8-bit pixel depth, 512x512, uncompressed tiff format preferably. Is there a canonical persistent URI someplace?
Only sort of. Since the image has been in use since before web services etc., more than one different copy was used as "canonical" so it's sort of unsolvable if you want to reproduce work. There has been more recent efforts to pick a reference one, as linked in this thread.