Wordpress - Alternative to using get_avatar function?

The get_avatar() function applies a get_avatar filter hook, that you can use to change the avatar markup:

return apply_filters('get_avatar', $avatar, $id_or_email, $size, $default, $alt);

I think this would be the correct way to hook into this filter:

function mytheme_get_avatar( $avatar ) {
    $avatar = '<img src="<' . get_template_directory_uri() . '/images/authors/' . get_the_author_ID() . '.jpg" alt="' . get_the_author() . '">';
    return $avatar;
}
add_filter( 'get_avatar', 'mytheme_get_avatar' );

EDIT

p.s. a nice alternative to this approach might be the Simple Local Avatars Plugin.

EDIT 2

The filter is applied using add_filter(), not apply_filters(). That was a typo on my part; fixed now!

EDIT 3

I don't think this is correct:

P.S: Just to clarify.. I replaced get_avatar($tc->comment_author_email, $jmetc_options['avatar_size']); with add_filter('get_avatar', $avatar, $id_or_email, $size, $default, $alt);

First, you still call get_avatar() in your template file, passing all the same parameters as previous. The add_filter() call belongs in functions.php.

Second, you can pass additional parameters to your filter function; e.g.:

function mytheme_get_avatar( $avatar, $id_or_email, $size ) {
    $avatar = '<img src="<' . get_template_directory_uri() . '/images/authors/' . $id_or_email . '.jpg" alt="' . get_the_author() . '" width="' . $size . 'px" height="' . $size . 'px" />';
    return $avatar;
}
add_filter( 'get_avatar', 'mytheme_get_avatar', 10, 3 );