1) Go to Settings (up right corner of page)
2) Click the "Edit Signature" link on the left navigation bar
3) That is where you can edit your sig (i.e. reduce the text size, add an image, etc)
4) To add an image from another side like in your case, simply click the "Add Image" button. It's to the right of the globe button(s).
5) It will ask you for the URL of the image.
6) Click the "Ok" button.