Check out example codes for "how to add site logo in wordpress". It will help you in understanding the concepts better.

Code Example 1

function themename_custom_logo_setup() {
 $defaults = array(
 'height'      => 100,
 'width'       => 400,
 'flex-height' => true,
 'flex-width'  => true,
 'header-text' => array( 'site-title', 'site-description' ),
 );
 add_theme_support( 'custom-logo', $defaults );
}
add_action( 'after_setup_theme', 'themename_custom_logo_setup' );

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