How can you see a photo of people that is not your friend in Facebook?

On there are certain privacy settings that may make it difficult to view some photos of others. However, if you have a mutual friend that can see the photo, they can always send you a link. If they just send the link of picture on Facebook, you'll probably get an error message saying you can't view that photo. But if they "right click>View Image" the photo, they can send you a link to that, which anyone can see. Nothing on Facebook is completely private, ever.