IANAL, of course, but I looked at the TOS again and it doesn't assign the rights to TwitPic. It just grants them the (non-exclusive) rights to use & sublicense. So they can use, they can sell, but they can't go after you for doing so via other channels.

It's still shady that they can resell your work in commercial channels other than the TwitPic service itself, but it's not like they obtain total control and ownership of your pictures.