how replace the widget title with a logo

Hello, the widget title is defined by the theme you’re using. For example, in “Twenty Fifteen” theme it’s defined in functions.php file:

function twentyfifteen_widgets_init() {
	register_sidebar( array(
		'name'          => __( 'Widget Area', 'twentyfifteen' ),
		'id'            => 'sidebar-1',
		'description'   => __( 'Add widgets here to appear in your sidebar.', 'twentyfifteen' ),
		'before_widget' => '<aside id="%1$s" class="widget %2$s">',
		'after_widget'  => '</aside>',
		'before_title'  => '<h2 class="widget-title">',
		'after_title'   => '</h2>',
	) );
}
add_action( 'widgets_init', 'twentyfifteen_widgets_init' );

As you can see, the widget title is h2, not h3.

Also, editing the theme you could be able to define an image.

thank you aldolat. i’ll try to find a solution for this

You’re welcome!