Most of us (okay, virtually all of us) have tons of digital photos floating around -- many are on our computers, some are on photo-sharing sites, some might even be printed (imagine that) and hanging on a wall or pasted in a book. But what happens if your computer, or your photo-sharing service, goes away? What happens if you save the images on a proprietary format, then want to look at them in 40 years, but that format can no longer be read by computers of the time?
The answer is complex, and deserves a longer article, but it boils down to these elements:
1. Store photo files in a simple, open format. The simplest "format" is an archival grade paper print, since you know paper will be readable for a longish time, and you don't need a computer to do it. Other formats like TIFF and JPEG are good choices, since they're likely to be supported in the future (and a TIFF- and JPEG-reading programs could always be written in the future, since the specification for the file formats are widely available)...but the future lasts a long time.
2. Store photo files in multiple locations. What if your house burns down, and your carefully collected hard drive melts? Make an offsite backup, either by literally copying stuff to a hard drive and mailing it to your friend; or by using an online backup service (I've used both Mozy and CrashPlan). Note: online photo sharing sites like Flickr may count as a place to store photos, but you can't count on these existing forever. Remember Geocities?
3. Store photo files on different kinds of media. This is the hardest one, and you might skip it. The thing is, it's unclear how long things like DVD-R discs will last, once they've been burned. I have CD-R's from 1998 that are still readable, but a few that are not -- that dye in a burnable disc basically melts over time; and then there's the problem of finding a computer that will still read an optical disc down the road. Today, optical drives are still common. Will they be in 40 years? (Remember floppy drives?) Similar issues exist with hard drives -- what if your hard drive is exposed to a big magnet, or is accidentally dropped? Your data may be toast. So ideally you'd have different kinds of storage for your important files, to insure that at least one of them survives.
Below is a video from the Library of Congress's "Personal Archiving Day" discussing some simple ways to archive your own digital photos. Check it out, if your photos are valuable to you.