Hooks

function streamtube_child_my_custom_content(){

    if( ! is_embed() ){
        ?>
        <img src="https://placehold.co/600x200">
        <?php
    }

}
add_action( 'streamtube/single/content/before', 'streamtube_child_my_custom_content' );

add_action( 'streamtube/single/content/after', 'streamtube_child_my_custom_content' );

add_action( 'streamtube/player/before', 'streamtube_child_my_custom_content' );

add_action( 'streamtube/player/after', 'streamtube_child_my_custom_content' );