You can already do that. Just get the location of the picture, and use that.
In Firefox, right click the picture and select "Copy Link Location". You can then paste the url into the box on your setting page.
In Opera, right click the picture and select "Copy Link Address". Then proceed as above.
If you use IE, go get another browser

Failing that, right click the picture and click "Properties". You then have to find the url on the Properties dialog box and copy it manually.
As for Safari, Konqueror, Chrome, [insert other browser name here], I have no clue. I only use the first two browsers on a regular basis (and IE when I'm held at gunpoint). Maybe someone else who uses other browsers regularly would know.