It is best when you start, if you start with the blocks stacked.
When you drop the blocks notice the Black Position Line, it tells you where the block is falling,
You can remove the spacers or line blocks or leave them, it is optional. But this is what you need when adding a new image block or positioning it.
Spacers or line blocks also help compensate for a 1 or 2 image row.
Now the important bit!
--> Like a Puzzle, push a block down in every row. As in the beginning add the block alongside the entire width of the page and then position it. The spacer lets you align it perfectly. See the difference here: