firstly, this site IS all about the preservation of the magazines and making sure they're available.
as far as "ownership" of scans or whatever, and someone can correct me if i'm wrong, but i believe that anything that you scan you are free and clear to upload wherever you want and do with as you please. if you wanted to post it here and at archive.org, there'd be no problem with that. some people prefer to only have scans (or whatever) on sites that they support and request that it stays certain places (which is why you may find a scan online for a magazine but it's not available here). But if it's a scan that you yourself are scanning/uploading, you can do whatever you want with it.