To have a banner you need 15 posts. Then, when you see a banner you like, click it and it should take you to the website where they created it. You should be able to create one at that website.
When you have the URL for the banner you want, you can add it by clicking 'settings' in the top right corner of the page of ECF, then edit signature on the left of the control panel. Should then just be a case of pasting in the URL to signature picture section.