You have the correct link jim, but you need to go to your user control panel...
Which you should be able to get to by this link: ucp.php?i=profile&mode=signature
If not, the in the menu go to User Control Panel --> In the left hand menu on that page go to Profile --> then click the "Edit Signature" link.
To add the image there, you'll need to use the "image" codes which is:
Simply insert the URL between the image code like this
There you go, hope that helps.