Hmmm...
It looks like Tumblr changed again, my only concern is that it does not look like their is an option for how multiple image post are arrange. And I normally like to try to make use of this format for my comic updates.
There is an option.
You actually have to drag and drop the images in any order you like. Or move them up or down to create a new row (that puts emphasis on that one pic).
It’s a bit cumbersome and hard to get used to, though. :/