Before posting to Usenet or to a computer mailing list, make sure that your message is in plain text. If you haven't made any changes to your e- mail settings, you're fine. Webtv's default setting is plain text. In the case of Usenet, it's because Usenet is a text-based system. Also, your message can crash older systems which are not set up to accept pictures and music in Usenet.
Mailing lists are also nearly always text-based.
This is to keep message size and loading time down, and so as many people as possible can participate without having to worry about system compatibility and message size. Most mailing lists will simply not let your message through if it has any elements other than plain text. Webtv lets you have up to six usernames at a time at no extra charge. Then, if you don't change any of the mail settings, you should have no problem posting anywhere. If this doesn't work for you, the next-best option is to decide that you can live without your pictures, music and other stuff and delete your signature.