Create your alternative text and convert it to a movie-clip, make sure that the text is positioned such that teh cross it at the bottom left of the movie-clip when you edit it (if you want to keep with tradition). Drag an instance of this movie-clip onto the main stage and drag it off the page so that the user can't see it, give it an instance name like "alt".
Now, convert your image to a button symbol and insert the following actions on it:
On (Roll Over)
Set Property ("/alt", Visibility) = "1"
Start Drag ("/alt", lockcenter)
End On
On (Roll Out)
Set Property ("/alt", Visibility) = "0"
Stop Drag
End On
OKAY
DAVE dave@davdesign.co.uk
^^^^^^^^^^^^^^^^