Social media 101 states that if users only want their friends to see their photos they need to set their account to private. But in case you haven't changed your privacy settings, it's probably not just old high school friends that are secretly studying your account.

Digital marketing companies are searching, studying and storing images from popular photo-sharing sites to collect data to be used by advertisers.

The marketing companies get access to the photos on Instagram, Pinterest and Flickr by using software code application programming interface (API). While the brands get their hands on users personal data for target advertising, the photo-sharing platforms are hoping that the big-named brands will hand over big bucks to advertise on their sites.

Some marketing companies like Piqora Inc. store photos for months to show potential clients what is trending. Others scan the selfies to identify logos of brands and the person's expression in the photo. This data is then used for target ads. 

As conversation about privacy continues, many criticize the social sharing sites for not letting users know that their personal selfies and other photos could be downloaded for marketing purposes. But the sites say they inform all users that publicly shared photos could be shared with partners.

The Federal Trade Commission requires sites to be transparent when sharing data with third parities. However, the rule is open to interpretation, allowing for downloading images in bulk.

"This is an area that could be ripe for commercial exploitation and predatory marketing," says Joni Lupovitz, vice president at Common Sense Media, a children's privacy advocacy group. "Just because you happen to be in a certain place or captured an image, you might not understand that could be used to build a profile of you online."

20 billion photos have been shared on the selfie-sharing app, Instagram. Users post about 60 million photos each day.