Actually, looks like the way to do it is to add this…
'img' => array(
'src' => array(),
'alt' => array(),
'title' => array(),
'height' => array(),
'width' => array(),
),
…to the $allowedtags array in /wp-includes/kses.php
Seems silly that this has to be done with a hack when the feature has been requested many times in the forums. : )
-Ryan