Aligning Block Content
You can align all the block's content - text and images, by clicking on the alignment button in the block's toolbar.
You can also do this from the inspector by going in the Block Tab > Alignment, then use the Content Alignment option.
The above methods change the alignment of all the blocks' content at the same time.
If you would like to change the alignment of the individual block elements, you can do this by clicking the individual blocks and adjusting the alignment through the Toolbar or Inspector.