leadegroot - 9:51 am on May 5, 2011 (gmt 0)
Not sure about the alt txt, but I have always moused over ads and then looked at the status bar at the bottom of the window to extract the url as a parameter. Mostly works on text ads - sometimes the url is too long and I can't see the appropriate parameter - I suppose I could widen the window.
Flash ads it doesn't work on. I don't remember about images right now (its not early)