It has come to my attention that if your photo's "server" number is 35 and you try to use the depreciated image URL format (i.e. "http://photos35.flickr.com/…." it will fail to resolve that server. You should use the new url format, which is:
http://static.flickr.com/{server-id}/{id}_{secret}_[mstb].jpg
If you're using phpFlickr::buildPhotoURL(), then you'll need to fix the format for that. In phpFlickr.php, you can replace line 262 with:
$url = "http://static.flickr.com/" . $photo['server'] . "/" . $photo['id'] . "_" . $photo['secret'];
Note: The old, depreciated method is supposed to continue to work, but there's apparently something wrong with the DNS resolution for that server. You may or may not be running into this problem and I imagine that it'll clear up at some point.