Images uploaded to Twitter/Photobucket seem to have a half-life. Images that were perfectly fine sometimes start returning a 404 image.
Some sizes of the image can be affected while others are not.
See:
http://pic.twitter.com/m1o6YLjs
http://pic.twitter.com/frB4h2nZ
http://pic.twitter.com/FyFGKqtg
http://pic.twitter.com/zkpK9s6W
http://pic.twitter.com/6KEZ29Si
http://pic.twitter.com/VgE7qbzn
http://pic.twitter.com/5ogjiC9d