Right click and copy the link address. Then you would go to settings (you need an account to see links) And go to Edit signature (you need an account to see links) Then go to the bottom and go to the 'Upload Signature Picture
Option 1 - Enter the URL to the Image on Another Website:' then hit upload, go to the top, add stuff like text if you want, and then hit save signature.
Yeah, all you do is cut what you have in your siggy and get the other one in there (the same way you did the first) then paste the first one back into the text field.