I get MMS messages regularly, and I've figured out that if you get a password and message ID error, it's because the picture is not "ready" yet. I think it was just a coincidence that the capital letters in the URL worked, because just last night I had the same problem and just waited a minute or two and it worked.
I will say the most annoying part about using viewmymessage.com is that it's difficult to check MMS messages while you're on the phone. I can't wait till they add native MMS support.