I am marketing specialist in Free Likes Club - a website for free Instagram likes. It is still in beta but its core feature is working and it is fast!
As on other social networks, users on Instagram can choose to “like” your content. They can do so by double-tapping your photo or video on the mobile app, or by clicking the heart icon under your content on the app or on a desktop. Instagram users do not have to be following you in order to like your posts.