How was that done?
Very simple: in the Tool line of the editor you can see grid symbol. Click that, this will insert the following:
<div data-theme-tiles="1">
Add images
</div>
Now all you have to do is to drag and drop the images you want in your image gallery between the
and
and the rest will be done for you. Please give the system a few seconds for your pictures to show in your preview window before you send the post.
Nifty, huh?