You can't. There's only one URL field you can use, and it can be a link or an image (or video).
When you post a link, the server will try to fetch the metadata for it (title, description, thumbnail image, and/or embedded video). So what you're seeing on posts that have a URL and an image are the thumbnail images provided by the metadata fetch of the link that's posted.
Workaround is to put the image in the body with the inline syntax: ![Image alt text](https://image.url/image.jpg)