currently there are many listeners for buttons (images, links) that set input values
but there are change listeners on theses inputs that modify the htmleditor

that means, that selecting an image : read and write all its attributes...

listener()
{
    var currentImg = ...
    altInput.setValue(currentImg.alt);
}

should becomde

listener()
{
    var currentImg = ...
    altInput.suspendEvents(false);
    altInput.setValue(currentImg.alt);
    altInput.resumeEvents();
}