Ah. I see what you're talking about. Yeah I don't have a way to do that. It'll be nice when that functionality is built out.
What I was referring to is when you click "Choose File..." you can paste an image URL in that and it will work as if you selected a file from your computer.
Team Mastodon over here. Best part is I don't have to explain why a federated social media is the way to go on Kbin.