It is best when you start, if you start with the blocks stacked. For the future, you can skip that, to save some work, but you will also need to use another tip I am showing here. I am going to use liner blocks to help me align the blocks. 

When you drop the blocks notice the Black Position Line, it tells you where the block is falling, see it now!

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. see it now.

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: