Remember you can search for mini tutorials and examples in the Webflow University pages…
Positioning - looks like centered is what you are after…Here’s Flexbox:
To pull a main image, you need a collection list, and then to add an ‘image’ inside the list - on image settings you can choose to pull an image from a CMS field. As long as the CMS item has an image you can choose the name of that field to populate your on page image.
Examples of the structures of what you are looking for, including individual blog pages, and blog lists are demonstrated here:
And a more general explanation of CMS/Dynamic content here: