Here's how to swap a default or existing image in a section for one of your own.

Move your mouse over the image and you'll see a green button.  

Clicking the button will open a media panel that shows the existing image or images in the section.  There's a representation of how the image or images will appear on "wide" screens (like desktop computers) and how they will appear on smaller, "tall" screens such as phones.

To change an image, click on it and choose a new one from your computer. 

It's possible to upload many types of images (read image guidelines here to find out how to optimise your images and make sure they fit the space well on differently-sized screens).  

A single image may naturally work well on both wide and tall screens.  If not, it's possible to shift the focal point of your image so that important parts aren't cropped out, or upload different images specifically for each screen format.