Adding graphics and additional formatting to your signature requires using an advanced editor outside of Outlook. To do this you would click Advanced Edit. When a dialog box appeared telling you that an editor not part of Outlook will open, you would click Yes. That's what you want.
Note Before you create a fancy signature, it's a good idea to check how you send e-mail messages. If you or most of the people you send to prefer Plain Text, a plain signature is all you need, and all you can send.