First, open your account of Instagram and go to the account where is the photo.
Double click on the photo and then click on the date, see the example with this photo Instagram on the verge website.
You need to click on7 hours…
This will open into browser adressbar this link: https://www.instagram.com/p/B-PQ743DNBe
Add to this link this: media/?size=l.
The new link: https://www.instagram.com/p/B-PQ743DNBe/media/?size=l will show you the full image of the photo.
Good to know!