Sometimes you select too big a photo for your Section field. It will look. unfit, especially on mobile view. To fix this, click on the photo in your Section field properties, and select the image icon from the menu.
There, you can set the photo's width and height. Set the width to 100% and save. It will ensure your photo's width is 100% of the screen size.
You can also watch the steps in the "Add an image" part of the following video: