Setting a 'feature image' for a post is important if you want a preview image to appear on the home page (This is exactly what I wanted), but... unfortunately by default that 'feature image' will then appear full width at the top of your post. (This is not what I want.)
Remove the feature image from the post
Using code injection it is possible to completely remove the feature image from the post.
Resize the feature image displayed within the post
Using code injection again, it is possible to resize the feature image and keep it centred.