Scan them. Now. Even relatively low end cheap scanners can capture more than the visible detail of a picture. Once you have a good digital representation, make a few backups on as long-lasting media as you can find, and put them in separate places. Give a copy to several relatives dispersed around the globe.

I know this doesn't preserve the original paper version, but it does preserve the pictures and the memories themselves. I understand the actual paper may have some nostalgic value, and the old "authentic" feel is pretty cool, but preserving the actual picture information should be the first priority. Note that none of this prevents you from then still doing the best you can to preserve the original paper. But, that will inevitably degrade over time. Note also that the paper prints are most likely not the originals anyway. Those are probably negatives that are long lost in time.

Aside about old family photos

Another problem with old prints is that there is only one copy. A picture of your great-grandfather, for example, is just as relevant to you as probably a whole load of cousins. The single picture had to be passed down to someone, and you ended up with it somehow. Now with today's technology, the cousins to whom it is just as relevant can all have access to it.

I inherited a bunch of such pictures when my mother passed away a couple of years ago. She did a lot of geneology research and collecting of family information, but it's all on one-off pieces of paper. I see my job to take what she collected and make it accessible to everyone. I have the single copies mostly by chance, but they belong just as much to a large number of other people.

I did some digging and found a web site that is trying to build the big One Tree of Everyone. It's totally free and open. See http://www.wikitree.com. This seems to be to be a good place to upload pictures of relatives to, record and save stories and life histories of, etc.

Another advantage of the one big tree is that after you get far enough adding the little branch you know about, you will be connected to information others have already entered. All of a sudden you are linked to ancestors that lived hundreds of years ago, especially if they lived in a place that kept good records. For example, when I got to my grandfather 6 generations back (turned out to be a state-sponsored pirate during the revolutionary war), I found he was already in the system and I was suddenly connected to ancestors that lives in the 1300s in England. One guy was a personal friend of William the Conquerer and faught with him at Hastings in 1066 and was rewarded with a large chunk of land. Pretty cool.