I'm receiving the amazing newsletter from Litmus and noticed something great.

For example, here you can see a alt text replacing the image (this I understand well):
with image blocking activated

But when we usually do this on newsletters, the space taken by the image is large, it's the same as when the image is loaded (after images blocking is deactiavted) even if there's no image.

For example
example where images take usually large space

How does Litmus manage to tell the email clien "hey here is my alt, but i don't want you to take empty space for all the image size? Indeed you can notice below that Litmus image, becomes a very arge image when image blocking is deactivated. So how did they manage to have the alt text take so little space ? It's very cool because enable not to have huge "empty spaces" inside images.
with image blocking DEactivated