When you make a public post on Instagram, you allow Instagram to sublicense you photo to anyone using the Instagram API.
One professional photographer found out about this the hard way.
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Unquestionably, Instagram’s dominance of photograph- and video-sharing social media, coupled with the expansive transfer of rights that Instagram demands from its users, means that Plaintiff’s dilemma is a real one. But by posting the Photograph to her public Instagram account, Plaintiff made her choice. This Court cannot release her from the agreement she made."
See Sinclair v. Ziff Davis and Mashable