what's the exact link you are trying to use? I can't help troubleshoot if you don't send it
Few things:
1. That link does not lead to an image, it's just the imgur home page, so it's definitely not going to work as a custom skin
2. You cannot use links that are blocked on your computer, they will load for everyone else but not for you. You can use an image hosting website that is not blocked on your computer if you want, pretty sure GitHub raw would work.
EDIT: GitHub raw does not work, but https://imgbb.com/ seems to. Here's the link I used which worked: https://i.ibb.co/sKNZv8V/eyes.png