You may get a "Transport Error" if your message could not reach the messaging platform.
*Power cycle you phone.
*You should change the image size to something smaller from the main Camera screen by selecting Menu and then Image Size. Once the image size is changed, you should try taking a new picture and sending that picture in a new message.
NOTE: Sizes for existing pictures can not be changed.
You may have to wait about 15 minutes after powercycling or try in another area.
If the error continues after waiting or in multiple areas, contact your provider.