Third-party image-hosting services can be...less than reliable sometimes, to say the least. Just last month there was a HUGE panic when Imagebam and Imgbox, two very popular hosting services (I frequently use Imagebam myself) announced they were shutting down at the end of the month. Fortunately, a few days prior to the witching hour, they found new server arrangements and are able to continue operations - however, a lot of thumbnails and such got broken in all the kerfluffle.
