Code snippet to add a custom plugin icon to the WordPress plugins update page. Icons were added to this page in WordPress 4.9 and are automatically shown for plugins hosted in the WordPress repository, but not for plugins hosted/updated elsewhere.

plugin.php

<?php

add_filter( 'all_plugins', 'prefix_add_plugin_icon' );
/**
 * Add a custom plugin icon to the plugins update page (introduced in WP4.9).
 *
 * @param $plugins
 *
 * @return mixed
 */
function prefix_add_plugin_icon( $plugins ) {
	$plugins['plugin-directory/plugin-directory.php']['icons']['default'] = plugin_dir_url( dirname( __FILE__ ) ) . 'css/images/icon.jpg';

	return $plugins;
}