The Montana stuff could be donated to the Museum of the Rockies or to the state Museum in Helena where Warren McGee's stuff is, might add a current perspective to his collection. Museum of the Rockies has the Ron Nixon stuff. The problem with scanning to DVD's is that they (the DVD's) have a limited life span--they don't last forever, especially the ones you do yourself--and the technology to read them might not exist very far into the future. Hate to say it to the computer geeks out there, but good old film still lasts the longest and can still be read, viewed and scanned in a variety of different ways---some of which don't require a computer. The on line archives are a good idea provided they don't go out of business as one did here a month or so ago (it was a non rail one that I can't recall the name of, but they flipped the switch when the money ran out and it was all gone--about 10 million images by one estimation.) Having them in multiple places and multiple formats would be a good idea.
This is a tougher question than you might think, many fine collections have been lost when someone dies and the family thinks all that "junk" should go in the trash (know of one fine toy train collection that went in a dumpster--those who know say it was worth in the seven figures---because the wife thought it was just a bunch of old toys). Write down what you want to have happen to the stuff when you die, and pick people that you know will carry out your wishes. (Also, if you plan to donate to a museum, ask the museum first---so they don't toss it in a dumpster because it doesn't meet their needs or plans). Also, there is no such thing as a worthless photograph, art types like one kind, modelers like another kind, and historians like still another kind. I've always been impressed at railroad shows as to what slides sell and to whom. Sorry to be so long winded, just finished the sad experience of disposing of a friends estate of fully restored antique tractors, the left over "junk" in the shop and barns--all rusty and ugly (that his kids wanted to dig a hole and bury) went for $490,000 at auction last month. Hope this provides some food for thought. (By the way, yes, in many cases the junk really is just junk, but sometimes you just don't know.)