I'm not sure I follow your response correctly, but it seems like your response is: it's easy to switch image sizes this way. But you still have to edit the HTML, and it's not that hard to edit the width/height. Maybe I don't understand?

The more important point to my mind is that grey is definitely not lorem-ipsum. It doesn't help me evaluate anything except image size. And since it's easy to get a sized placeholder, a server to generate a URL seems not that helpful.