Any of you has experience with some type of printing that guarantees a long preservation? Not necessarily >100 years, but at least several decades? (I am in my 40s, and I seriously plan to stick around for the next 50 years or so).
Use pigment based inks on an acid free paper and store the prints using archival methods.
Less expensive inkjet printers tend to use dye based inks. They produce amazing prints and the prints can have a long life. But dyes often break down over time (this can be a problem with color prints produced photographically as well).
There are pigment based inkjet printers. They tend to be more expensive and a bit temperamental unless used all the time. Pushing tiny bits of pigment through inkjet nozzels is harder because there must also be a binder. Small hole, grit, and glue is not a recipe for success but the printers do it anyway.
Pigments tend not to break down over time and that's why we can still see cave paintings and other pictographs.
All that said, keep the originals.
Did I say keep the originals?
If not, keep the originals.
The only reason this question has come up is because the originals are still around.
Good luck.
A colleague of my father made some etched copper plate images from photos. I don't have his full process, but he photocopied the original photos a few times to get an image made from black and white regions. This he photocopied onto a transparency, which he then used to etch a copper plate using a circuit etching kind of workflow.
The final plate had smooth or matte copper areas corresponding to light and dark.
Quite striking and I guess it should last a long time.
E.g. some good inkjet inks are claimed to last 200 years, on good (acid-free) paper and stored well those should last quite a while.
Another great advice is to make a high quality scan. High resolution, full color. Output it to TIF file format. That should give you a file that you can print anywhere.
So you need to take care and archive the printed one _and_ the digital one.