Write4net is a separate service to twitter, it allows you to compose a blog, meaning you can insert pictures, videos etc. It publishes it to your twitter account by converting the blog url into a small url (something like TinyURL), your followers receive the twit as a link which when linked takes the users through to the original posting on Write4net.
Write4net is a free service and is very easy to use. Basically when you link through to the Write4net website, you land on the compose page. You type whatever you want and then click on the publish link. It will then ask you for your twitter username and password, allow access and you are ready to go.
It’s like blogging in micro-blogging, I like that!