LinkedIn image size is the same as Facebook image size when you share a website link. It’s 1200×628. Usually it would be a logo image. You would place it in the header using the “og:image” property. For example:
<html prefix="og: http://ogp.me/ns#">
<head>
<title>The Rock (1996)</title>
<meta property="og:title" content="The Rock" />
<meta property="og:type" content="video.movie" />
<meta property="og:url" content="http://www.imdb.com/title/tt0117500/" />
<meta property="og:image" content="http://ia.media-imdb.com/images/rock.jpg" />
...
</head>
...
</html>If after you added image successfully, cleared your website cache and it still don’t show, you can go here to preview it and after you preview it and it looks good, it’ll work on your LinkedIn posts: https://www.linkedin.com/post-inspector/
I referenced this article: https://www.linkedin.com/pulse/how-clear-linkedin-link-preview-cache-ananda-kannan-p/
For Facebook, you can test here: https://developers.facebook.com/tools/debug/