Well the full URL will be your current URL location, minus the last .html or .php etc file name, plus /images/myimage.gif.
So on example.com/blah.html
the mentioned URL would be
example.com/images/myimage.gif
If you're trying to get firebug itself to expand and show the FULL url, you can also right click in the inspector and choose "show image location". But that's one way to do it.
Another way is to right click the image, and in Chrome you can choose "Copy image URL" which would give you that full address. Just checked - Firefox also has "Copy image location" which would do the same thing.