Hrm, that sounds like a bug, as I don't think it should have shown on the global screen ever, except for the default list that comes with the plugin. Can you open a bug report for it, so I can see why that is happening?
This part is not a bug because the reason that it should up and should have shown up at the beginning is because I added "path.jpg" => "path.jpg" to the default list that comes with the plugin.
Where I would think it is a bug is that if I have added it to the default list in plugingravatar.class.php, then it should up in the global setting dropdown list. However it does not unless you go into class/view/plugingravatarconfigview.class.php and change the default to be path.jpg.
The problem with the above solution is that if you added several images in the default global list, how would you be able to get the rest of them to show up in the global setting?