Add something like:
15 15 * * * php /Library/WebServer/Documents/magpie/flickr.php
to your crontab, and it will run once a day, fetching the RSS feeds and downloading all the new pictures to a folder, which you can then set as your Desktop Pictures folder. Set the picture to change every 15 minutes, and make sure it's centred not stretched.
define('MAGPIE_DIR', '/Library/WebServer/Documents/magpie/');
require_once(MAGPIE_DIR.'rss_fetch.inc');
$feeds = array(
'http://www.flickr.com/services/feeds/photos_public.gne?tags=garden&format=rss_200',
'http://www.flickr.com/services/feeds/photos_public.gne?id=59904640@N00&format=rss_200',
'http://www.flickr.com/services/feeds/photos_public.gne?id=51035609472@N01&format=rss_200',
'http://www.flickr.com/services/feeds/photos_public.gne?id=54289096@N00&format=rss_200',
'http://www.flickr.com/services/feeds/photos_public.gne?id=49503155549@N01&format=rss_200'
);
foreach ($feeds as $url) {
$rss = fetch_rss($url);
$items = $rss->items;
foreach ($items as $item) {
$description = $item['description'];
if (preg_match("/www.flickr.com\/photos\/(\w*?)_m.jpg/", $description, $matches)){
$img_url = "http://www.flickr.com/photos/$matches[1]_o.jpg";
$filename = "/Users/YOUR_USERNAME/Pictures/flickr/$matches[1].jpg";
if (file_exists($filename) == 0){
system ( "wget $img_url --quiet -O $filename" );
}
}
}
}