There are some issues with Yuku about turning URL's in to links automatically. Sometimes it works and sometimes it doesn't. But i have found an explanation about how to post a link here: http://jamesmcavoy.yuku.com/reply/15651#reply-15651

About the signature: did you change something about it? Or did it comes this way? (If so that is a new Yuku thing as well) but if you changed it yourself, the words 'Your Signature' may have already been in there to show you where it would go and when you added the 'Hi There' with the banana you simply forgot to delete them.

There are more explanations about making a signature here: http://jamesmcavoy.yuku.com/reply/5155#reply-5155

Hope that helps!