How to change size of image in relation to text on a post?

A hopeful easy one...
How do I change the default margins on a post template so that the image stretches all the way across the page, but the text is centred and has wider margins. I'd quite like to move the heading too to sit over the image.

Example here https://adventure.com/mexico-isla-maria-madre/

And what my post currently looks like
https://ethicalescapes.org/responsible-travel-in-thailand