Could you leave out the alt="" tag? What does it stand for when it's not specified? That the image will have original alt?
Nothing will be displayed when a user put the mouse over. This is not necessary but it can work as tags for search engines
Alt is the alternative text or placeholder wich is displayed if there is an error loading the image.
Also, Robin's answer is correct
Yours too :)
Each browser will display a different error, even simply displaying nothing. Do you remember on IE7 the red "x" at the left top corner? :)
It's not compulsory but it's necessary coz if the browser used to open the page doesn't support the image there is an alternative to bring forth the information