WP admin custom logo without a plugin
// make sure to upload logo.png into the theme images folder first
// on functions.php:
function custom_login_logo() {
echo '<style type="text/css">'.
'.login h1 a { background-image:url('.get_bloginfo( 'template_directory' ).'/images/logo.png) !important; background-size: contain; width:100%; margin: 0;
padding: 0; }'.
'</style>';
}
add_action( 'login_head', 'custom_login_logo' );