From a user's perspective, Instagram has:
a) a way to post pictures/videos/sound recordings to a public feed. The pictures can include overlays of links to other users, to other posts, to song lyrics that play in sync with the music, etc etc. Users viewing their posts get the ability to comment/like/link, with automatic language detection and translation on demand.
b) a way to see how other users interact with their posts, allowing comments, seeing views and other analytics, monetizing etc etc
c) Provides advertisers with the ability to place stories (stories are a stream of short-lived (24 hours) video/audio posts that users see) or posts (that can be static/video/audio), with links to external sites, purchasing direct links ("Shop now"/"Buy this") etc
Instagram is much more than a stream of user images.
That doesn't include all the "back office" stuff like spam/reporting/censorship/language translation etc etc.
If you think it’s just a bunch of static photos in a feed, you’re wrong and it’d be clear you know nothing about the service.