Photos Cover - Access is Forbidden

In my photo cover album, I see some small thumb pictures which I can click on to see a larger picture, but some of them, I get "Access is Forbidden".  

My question is, if someone create a private Album, the small thumb pic should not appear at all, but why I can see the small thumb pic and yet I can't see the larger pic ?

How do I make all pictures that get published as "public" by default unless the user chooses to make his/her photo album private ?

Thanks ! 

It looks like it's default behaviour, can you reproduce the problem on demo.boonex.com ?