You are using Instagram, right? And you might have kept your posts private, so that only your followers can view your posts. Yes, even I have ticked the option to allow only my followers to view my posts.
That option works well if you are browsing through Instagram only. But what if you post your Instagram picture’s link like this:
The post on your Instagram profile was limited only to your followers (maybe 150, 1500 or 150k), but now your tweet has made that picture available to millions of people who are on the Internet. Anybody can click on the link and see your picture.
Although, when you visit my profile, you will see that my account is private!
Instagram should follow one rule- if an user opts to have a private profile, you need to make sure (from server side) that the posts remain visible only to the followers on Instagram.