This really Seems like the long way to get there.
1. Instead of using View source click on "Inspect" (for chrome) "Inspect element" (for Firefox)
2. It will Highlight the place in the code. you just need to click the Arrow expander to see the image links. Click the thing red arrow is pointing at to expand.
3. you will see an '<img>' tag. In there will be various sizes of the image for faster loading and screen sizes. Don't let it intimidate you, the best quality image is the very last one. just right click and open in then tab.
4. Then right-click and save. You're done.
Here is an example of the HTML image tag that Instagram uses. I spaced out the different size images so you can see its very simple.
HTML:
<img alt="Image may contain: one or more people, bedroom and indoor" class="FFVAD" decoding="auto" sizes="480px" srcset="
https://instagram.fphx1-1.fna.fbcdn.net/v/t51.2885-15/sh0.08/e35/p640x640/74341843_751890915325974_8005170848566861371_n.jpg?_nc_ht=instagram.fphx1-1.fna.fbcdn.net&_nc_cat=106&oh=a84f2c1d0b1d37f072cea02a89fd33ff&oe=5E7DAEF3 640w,
https://instagram.fphx1-1.fna.fbcdn.net/v/t51.2885-15/sh0.08/e35/p750x750/74341843_751890915325974_8005170848566861371_n.jpg?_nc_ht=instagram.fphx1-1.fna.fbcdn.net&_nc_cat=106&oh=751e99da336355bac1bdb694f4005e35&oe=5E8C2BF3 750w,
https://instagram.fphx1-1.fna.fbcdn.net/v/t51.2885-15/e35/p1080x1080/74341843_751890915325974_8005170848566861371_n.jpg?_nc_ht=instagram.fphx1-1.fna.fbcdn.net&_nc_cat=106&oh=8d21a028c4ae9f2b4610e1965fe9442e&oe=5E704C16 1080w"
src="]https://instagram.fphx1-1.fna.fbcdn.net/v/t51.2885-15/e35/p1080x1080/74341843_751890915325974_8005170848566861371_n.jpg?_nc_ht=instagram.fphx1-1.fna.fbcdn.net&_nc_cat=106&oh=8d21a028c4ae9f2b4610e1965fe9442e&oe=5E704C16" style="object-fit: cover;">