At the top of the page there will be a link you can click that says "User CP" for User Control panel. Click on that and it will take you to your personal control panel. Then on the left side of the page about in the middle, click on the link that says "Edit Signature". It's under the settings and options tab. Once in there you can type in what you want and upload an image URL or just upload one from your computer directly.

Hope that answers your question.

And of cource you got replies...that would be terrible if a new member didn't get any replies in their welcome thread.