C98’s Twitter-hosted photos show up fine for me. But it is best practice to not “direct-link” to photos on other sites (considered to be bandwidth theft). Host on a server you own or choose a photo sharing site like “Photobucket”, “Imageshack” or even “DropBox”.
http://deuceofclubs.com/switcheroo/index.html
http://deuceofclubs.com/switcheroo/switch09.htm
Actually, for Twitter their policy is to use the embedded tool and to keep the photos on Twitter. They don’t want you downloading and rehosting the photos.