Hi pipoulito,

Yes that’s possible

<?php

/**
 * Plugin Name:       Default Featured Image, apply to posts only.
 * Plugin URI:        https://wordpress.org/support/topic/default-for-cpt/
 * Version:           1.0
 */

function dfi_no_page_post( $dfi_id, $post_id ) {
	$post = get_post( $post_id );
	if ( $post->post_type === 'page' ) {
		return 1234; // For Pages set the image with media ID 1234 as the default.
	}

	return $dfi_id; // the original featured image id
}

add_filter( 'dfi_thumbnail_id', 'dfi_no_page_post', 10, 2 );

Adjust the image id (1234 in the example) and replace ‘page’ with your custom post type.

Let me know how it goes.

Jan-Willem