How do I align pictures underneath eachother?

Hi, that is a post and in the content area of that post, there are three images that have been inserted with an alignment of “none” and inserted at full size. They have also inserted a featured image (the Swan image at the top).

How do I make sure that when I get in a post, the big banner image (is that what you call a feature image) is fitting, where can I chose what picture will be in there?

In the right side of the post and page editor, down a little, you should find the Featured Image module. That is where you choose and apply a featured image for pages and posts. The featured image is the full-sized image that will appear at the top.

If you go to the Cubic Theme Showcase page, and scroll down to Quick Specs near the bottom, you will find the image size for featured images (2000px wide by 1500px high).

Okay, now I know that, but where can I those what picture will be in the boxes on this link: https://wordpress.com/theme/cubic (to the top right there are six boxes). If I get the feature image fitting, this one looks very weird ( with black bars top and bottom)

If you are talking about the images in the screenshot on the right of the showcase page, those are featured images set for the blog posts.

If you use an image that is 2000px x 1500px, or at least those same proportions, the theme software will center-crop the image for use on the page that shows the posts. If your images are not that size, or at least those proportions of height and width, they could very well end up with some blank on the left or right or top or bottom. Cubic works well with landscape oriented images (wider than high). It does not do well with portrait oriented images (taller than wide).