One component of a solution might be to make lower-resolution versions of the pictures (e.g. use ImageMagick to do a batch scale-down) and store those in multiple copies. You could probably reduce the size by 90% and still have perfectly useable pictures. Obviously lower-resolution is not as good as the original, but it would make it more practical to make many backups, and keep them in many places. Lower-resolution is better than nothing.
(This is not meant to replace backup of the originals - just make it possible to make a lot more, and as a result a lot better chance of something surviving.)
BTW - Make sure you also backup the captions for the pictures - you are going to do that, right? I've just spent the last few months going through my father's old slides, trying to figure them out.
BTW2 - I made a bunch of backups on CDs about 10 years ago. I went through them a few months ago - about 75% were unreadable.