Incorrect. Anybody can see the image you posted (this one:
[url]https://scontent-lga3-1.xx.fbcdn.net/v/t31.0-8/20231952_10155579122556340_8381249603268064593_o.jpg?_nc_cat=103&oh=d3626a399759521b70bfee81567d13eb&oe=5C19AABC [/url] )
It's possible that you've set this photo to "viewable by anybody" and that Facebook is verifying that setting before letting me view it. I don't use Facebook so I haven't bothered to check what they do, but some services don't check at all - images are stored in a non-secure part of the website, and they only authenticate when you view a page containing the photos, not when you use the direct URL of the photo.
(edit: some crude poking around reveals that Facebook does check that the URL is formatted in a particular way that makes it harder to find additional photos when you know the URL of one photo. Notice that the URL doesn't end with ".jpg" - there's some extra information that the server is using to verify that the request is valid. If you try to just view the directory containing your photo [which would be the easiest way to see a list of other photos], you'll get "invalid URL signature." Mess around with the various fields within the URL and you get various other errors. With some patience I could probably figure out the format, but it'd take me a while. That's not particularly reassuring - facebook has existed for years, and it's a royal pain for them to change their URL format, so there's almost certainly somebody out there who has "cracked" their format long ago.)