appending text to each alt tag jquery

appending text to each alt tag jquery

I am attempting to create a function using jQuery that appends a copyright
message to the end of all alt tags. So <img src="image.jpg" alt="this is
alt text"> would end up <img src="image.jpg" alt="this is alt text -
Copyright (c) John Doe">.
This is the function I created but it's not working:
$(document).ready(function() {
$("img").each(function() {
$img.attr("alt").append(" - Copyright (c) John Doe");
});
});
Can anyone see what's wrong?