I’m new to web dev, too. I tried to figure this out, but the best I could do was to copy the full HTML by using Inspect in Chrome. That gets you this, which doesn’t work, even though it’s basically the only HTML on the page:
<img style="-webkit-user-select: none;margin: auto;cursor: zoom-in;background-color: hsl(0, 0%, 90%);transition: background-color 300ms;" src="https://instagram.fbeg2-1.fna.fbcdn.net/v/t51.2885-15/sh0.08/e35/s640x640/211923633_341835957436071_2595781851662408146_n.jpg?tp=1&_nc_ht=instagram.fbeg2-1.fna.fbcdn.net&_nc_cat=103&_nc_ohc=upj99hCjROUAX93eUge&edm=ABZsPhsBAAAA&ccb=7-4&oh=bc90a021d242b91c19fbfb074876837e&oe=60EDD8F5&_nc_sid=4efc9f" width="454" height="454">
Isolating just the image link (https://instagram.fbeg2-1.fna.fbcdn.net/v/t51.2885-15/sh0.08/e35/s640x640/211923633_341835957436071_2595781851662408146_n.jpg) doesn’t work, either.
I think maybe Facebook/Instagram are using some kind of special link that can’t be shared outside the platform. My workaround would be to download the image and upload it to a free image hosting website and link to that.
Hopefully someone else can help.