html – Download an image with node.js after it renders

It is a bit complex, what I currently do:

1. I have an HTML-Page hosted on Github Pages
2. I go to that page with a urlParameter
3. The page parses it and renders a jpeg encoded with base64 (image.src=”http://superuser.com/data:…”)

What I want to do:

1. I send some kind of request to the page from node.js, it will be inside a discord bot. I completely have no experience with that. Also please note, that I can’t use something like Selenium.
2. Then I wait until the page loads/the jpeg image renders.
3. I want to download this image and be able to attach it to a discord message (This part I will figure out myself).

Could anybody help?
If the question is not clear enough, I will explain more in detail.