Acid free archival paper kept in a dark, fire proof box is to my knowledge the single best technology we've invented and easily at our disposal for long-term storage.
I think I might totally start 'printing out' photos I want to theoretically have available in fifty years and place them in a safety deposit box…
How stable would the various colors of ink needed for pictures be? I suppose you could also print out a separate B/W version of each color channel for later reconstruction.
Maybe you could print out a color calibration strip with known values, and then use that as a benchmark so you could reverse fading via software? Kind of like what NASA/JPL does so they can color-correct photos taken by planetary landers.
(I know that this isn't what the OP meant, but it's interesting to try and mitigate the limitations of the more common backup solution -- i.e., just printing the photos themselves).
Next to paper or parchment (mentioned below), there is a technique, that uses microform. Here in Germany a lot of long-term archival work is done with this material. The information is scanned and "printed" on microfiches. In one image, there can be stored a lot (i think up to 16) pages and a lot of images are on one roll of film. Then these rolls are transferred into special containers and put in an old mine in the German "Black Forest".
There are to date more than 28.000 kilometers of microfilm with round about 850.000.000 images. They estimate, that even in 500 years there will be no information loss on these images.
Very true. Acid-free paper has a expected life of over 1000 years. Think about how long paper documents like the Magna Carta, ancient illuminated bibles from the 700's, Shakespeare's plays have survived without special climate-controlled environments.
Also paper in bulk is hard to destroy. A stack of paper is pretty dense and can survive to some degree a house fire or flood.
I think I might totally start 'printing out' photos I want to theoretically have available in fifty years and place them in a safety deposit box…