How can I make my larger images expand to full width of the article section and display responsively on different screen sizes?
By default, a Hub Item's article section width is 840px maximum width and any image you upload that is 840px or wider, will display as full width of the article section and will automatically be responsive across different screen widths.
In a situation where your Hub design has the default Article section width set wider than 840px via custom code, you can edit the image width in the Item Editor>right click>image properties, and set the width and height of that image to 100%. This will allow the image to expand to full width in that wider than usual article body.
Just note that if you are going to set width of an image to 100%, and the original image is not as wide as the width of your Article body, that image can display with deteriorated quality because you are stretching the image beyond its original dimensions