here is a new question regarding this:

How can you lock a particular folder yourself? so that in the future, you can Unlock it for a specific user?

Though this is a rather old thread, I'll state what I have found so far about "locked" albums.

I could not find any "official" description about what the setting does, but a couple of messages of admins stating in an uncertain way that marking an album as "locked" will prevent it from being seen by users who haven't joined to any of the forums who the owner joined too.

Facing the constant issue in our forum of avatar and photos being deleted because of its content, it would then make sense to lock our album expecting our partners to be able to access them...
Well, unfortunately that doesn't work! After the latest raid of deletion which happened by the beginning of this month, I found most of my friends have been trimmed down (me too) and the few that re-uploaded their pics marked the albums as locked.

I posted a thread in the general help forum and I was answered that apparently the only effect of the setting is to prevent the albums from being accessed by clicking (and the images in them won't be played in the slide show). So the only purpose of the locked albums is to act as repositories of iles to be shared in threads.

I am a programmer and work in IT and I'll try to get in touch with a techie of the site to suggest if they could implement any way to allow accessing locked albums in an easier way.