Try changing the order of initialisation. Demo: http://jsfiddle.net/GCu2D/77/ . Replace this :
$(function(){
$('#img_logo').draggable().resizable({ aspectRatio: true }); //works
$('#img_header').resizable(); //doesn't work: shows the icon, but doesn't resize.
});
With:
$(function(){
$('#img_header').resizable();
$('#img_logo').draggable().resizable({ aspectRatio: true });
});