I’m not sure about what you really want. If you are looking for an image instead of a text in the header, you have to create a child theme, download the header.php file, upload your logo, and replace the bloginfo(‘name’) by your image file. Something like this :
Replace default site-title :
<a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a>
By this :
<a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><img src="you-image-path" alt="your-image-alt"></a>
