How to change the Default block in the block-editor away from the paragraph block?

You can use the Gutenberg Block Template it is used to have a content placeholder for your Gutenberg content.

https://developer.wordpress.org/block-editor/developers/block-api/block-templates/

<?php
function myplugin_register_template() {
    $post_type_object = get_post_type_object( 'post' );
    $post_type_object->template = array(
        array( 'core/image' ),
    );
}
add_action( 'init', 'myplugin_register_template' );