Interesting..
I've been thinking about that too.. Left my logon info somewhere for my wife but maybe there are better options..
Beyond leaving log-on info to stock sites, email attached to sites, and paypal (or however payment arrives), if we're really talking about decent money, person should play safe and ask
LAWYER to include giving copyright of images/ income from specific accounts to specific people in Will. I'm not a lawyer, but if will already leaves everything to one person, I guess you could be covered legally.
Also, tell new copyright/account owners to log in every X number of months. That way, it will be a habit, and they'll see any announcements that may affect their account.
I noticed on one of the sites that it's going to delete photos that haven't sold, of contributors who haven't logged in in over a year, so it would be risky to have a poorly attended account (where all one does is wait for payments to land) as the years go by.