Nope, both is possible, and now the case for this post. When you upload an image, Lemmy generates an URL for it and auto-inserts it into the URL field. You can then replace it with any other URL without the image getting deleted from the post. It's just that clicking on the preview of the image will then take you to that new URL instead of enlarging the image. In any case I find it confusing, the two things should be handled separately in the UI