When you send an image into the Webflow CMS API, it needs to be on a URL that is public, where Webflow’s API can download it.
In your case, it looks like the URL you’re giving is strange, it begins with webflow.com/api/sites …
So your first task is to figure out where your uploaded image data is being stored and how to expose that as a URL. Then pass that into the API as the image url.