Are you wanting to see the contents of the text and picture messages online? If so, that's not an option through your My T-Mobile account. We do not store the content of your messages on our servers. On My T-Mobile, you can only see the phone number, time and date they were sent to/from.
I know this is a long time after the post, but in case anyone wants to do this, and found this post on the forum, I suggest checking out using Google Voice.