Retroactively sideload images into WordPress from Flickr. Used for a once-off. Not lots of error checking. Stick in mu-plugins and then load wp-admin/?do_flickr_sideload

flickr-sideload.php

<?php

/**
 * Retroactively sideload images referenced from Flickr.
 */

$flickr_sideload_media_id = null;

function flickr_sideload_get_media_id( $media_id ) {
	global $flickr_sideload_media_id;

	$flickr_sideload_media_id = $media_id;
}

function flickr_sideload_init() {
	global $flickr_sideload_media_id;

	if ( isset( $_GET['do_flickr_sideload'] ) ) {
		add_action( 'add_attachment', 'flickr_sideload_get_media_id' );

		require_once( ABSPATH . 'wp-admin/includes/media.php' );
		require_once( ABSPATH . 'wp-admin/includes/file.php' );
		require_once( ABSPATH . 'wp-admin/includes/image.php' );

		$offset = 0;
		$limit = 10;

		while ( $posts = get_posts( 'posts_per_page=' . $limit . '&offset=' . $offset ) ) {
			$offset += $limit;

			foreach ( $posts as $post ) {
				if ( preg_match_all( '/(?<atag><a[^>]+href="(?<href>[^"]+)"[^>]*>)\s*(?<imgtag><img[^>]+src="(?<src>[^"]+)"[^>]+>)\s*(<\/a>)?/i', $post->post_content, $images, PREG_SET_ORDER ) ) {
					$post_content = $post->post_content;

					foreach ( $images as $image ) {
						if ( strpos( $image['src'], '.flickr.com/' ) !== false ) {
							$description_matches = array();
							$description = '';

							if ( ! empty( $image['atag'] ) ) {
								preg_match_all( '/title="(?<title>[^"]+)"/i', $image['atag'], $description_matches, PREG_SET_ORDER );
							}

							if ( ! empty( $description_matches ) ) {
								$description = $description_matches[0]['title'];
							}
							else {
								preg_match_all( '/alt="(?<alt>[^"]+)"/i', $image['imgtag'], $description_matches, PREG_SET_ORDER );

								if ( ! empty( $description_matches ) ) {
									$description = $description_matches[0]['alt'];
								}
							}

							$image_html = media_sideload_image( $image['src'], $post->ID, $description );

							preg_match_all( '/width="(?<width>[0-9]+)"/', $image['imgtag'], $width_matches, PREG_SET_ORDER );
							preg_match_all( '/height="(?<height>[0-9]+)"/', $image['imgtag'], $height_matches, PREG_SET_ORDER );

							if ( ! empty( $width_matches ) && ! empty( $height_matches ) ) {
								$size = array( $width_matches[0]['width'], $height_matches[0]['height'] );
							}
							else {
								$size = 'full';
							}

							if ( ! empty( $image['atag'] ) ) {
								$image_html = wp_get_attachment_link( $flickr_sideload_media_id, $size );
							}
							else if ( $size != 'full' ) {
								$image_html = str_replace( '<img ', '<img width="' . $size[0] . '" height="' . $size[1] . '" ', $image_html );
							}

							$post_content = str_replace( $image[0], $image_html, $post_content );
						}
					}

					if ( $post_content != $post->post_content ) {
						wp_update_post( array( 'ID' => $post->ID, 'post_content' => $post_content ) );
					}
				}
			}
		}

		die;
	}
}

add_action( 'init', 'flickr_sideload_init' );