At the top of the page, there is a grey head. Click it and a box should appear that says preferences. Click that. It will redirect you to a page, on the left side of that page (Under your profile picture) you should see a signature link, click it.
When you click it, it will redirect you to another page that looks like a reply box. At the top of the box there is an image box you can click, it says paste the URL. Do so and click enter or save and that should be it!.
Re: Sig.