Wrong file name. Now it displays correctly.
@ -56,7 +56,7 @@ function monero_create_menu()
'manage_options',
'admin.php?page=wc-settings&tab=checkout§ion=monero_gateway',
'',
plugins_url('monero/assets/icon.png'),
plugins_url('monero/assets/monero_icon.png'),
56 // Position on menu, woocommerce has 55.5, products has 55.6
);