How do I Add images uploaded in the post to a default custom field

Can you explain why you need a custom field for the image ? if there is no special reason for that , you should use the_post_thumbnail() function that is built in wordpress and will not have you mess with custom fields.

You can read more here :

Display thumbnail from custom field

EDIT –
Well, if you insist on using it :

add_action('wp_insert_post', 'mk_set_default_custom_fields');
    function mk_set_default_custom_fields($post_id)
    {
        if ( $_GET['post_type'] != 'post' ) {
        $image1 = wp_get_attachment_image_src(get_post_thumbnail_id()) ;
            update_post_meta($post_id, 'image', $image1[0],true);
        }
        return true;
    }

Of course the feature image needs o be defined (or set a default) , and you can choose the sizes in the get_post_thumbnail() function.