Your site is not hosted on WordPress.com, but using the open source WordPress.org software at another hosting provider. We cannot help with this in the WordPress.com forums.
For a custom header image to be used in the Facebook preview, that image will also need to be set in your site’s OpenGraph tags in the HTML headers. WordPress does not do this automatically, but you need an SEO plugin like Jetpack or Yoast SEO to configure that.
Please post in the self-hosted WordPress forums instead for more help with this, or if you already have a SEO plugin, contact support for that plugin directly: