In order to send HTML and images within WP email messages, use the “wp_mail_content_type” filter to set the type to text/html. If you call wp_mail() directly, you could instead supply 'content-type' => 'text/html', as part of the array passed as $headers.

In case you were not aware, email clients are terrible at properly rendering HTML in messages. Don’t try to do anything fancy or it will surely fail to render correctly in someone’s mail client. Stick to the very basics. Test your message in as many different mail clients you can to be somewhat confident your message will appear correctly.