donnapix.php

<?php
class DonnaPix {
	const PIC_IPSUM_URL = 'https://picsum.photos/';

	public function __construct() {
		add_filter(
			'wp_calculate_image_srcset',
			array( $this, 'srcset_filter' ),
			10,
			1
		);

		add_filter(
			'wp_get_attachment_url',
			array( $this, 'attachment_url_filter' ),
			10,
			2
		);
	}

	public function attachment_url_filter( $url, $attachment_id ) {
		return $this::PIC_IPSUM_URL . '600';
	}

	public function srcset_filter( $sources ) {
		if ( is_array( $sources ) ) {
			foreach ( $sources as $key => $s ) {
				$sources[ $key ]['url'] = $this::PIC_IPSUM_URL . $key;
			}
		}
		return $sources;
	}
}

$donna_pix = new DonnaPix();