This topic has been discussed a lot recently. Authors should provide different sources for images in different sizes, and based on their visual judgment, they will focus the main element of the image for that particular breakpoint. This is art direction.
Let me clarify it by showing this case here. When the image is displayed in larger sizes, it makes sense for the image to show the couple on the boat and the river in the background. The background helps explain where they are but, in general, it gives no relevant information. Now, look what happens when we scale the image down to fit a smaller screen. It is not art direction.
Reducing to that size, you can barely recognize the couple. Instead of simply resizing the image, it may make sense to crop it to get rid of some of the background and focus on it. The end result is an image that works better in a smaller size. Let's compare the left picture (art direction) and the right picture as follows...