Or "Anyone with the link." If you set it like that, people can't publicly search for the images, and can only arrive at the image if you use a link (with a significantly difficult identifier to guess) that's generated by Google and used by you wherever you see fit.

I don't see a specific "don't make me searchable" setting... could've sworn I'd seen one at some point, maybe they moved it, or removed it in lieu of the per-album settings.