As you can see, you can click and drag the Image Blocks into place next to one another, or you can drag the Block straight into place from the Blocks Menu.

Using a Grid Style Gallery Block will give you design options that allow you to remove the padding space between the images, which the Image Blocks option does not allow for.

Using the Grid Style Gallery Block may also be great if you've got multiple images you want to position in rows and columns.  You can toggle this within the Design portion of the Block's settings.

To modify the size of the Gallery Block itself, use Spacer Blocks.