I thought this was gonna mean private for members as well, which I would've definitely disagreed to. But yeah, I actually think guests should be blocked out. Nobody knows who could be watching and the fact that people you don't know are going over your pictures is a biT weird.

At least you can see when members are in the thread and (if they're not on invisible) see who they are, while you never know who a guest is and they're more likely to do mischievous things with your pic than a member would.