Hi
The images in homepage boxes are coming from that page featured image. First of all you have to add page. Follow this instructions.
Go to Pages >> Add new page >> add title, description and featured image after doing that you need to go to Appearance >> Customize >> Homepage Boxes >> here you can find that page you have just created then select that page and the title, description and featured image will be displayed of that page so whenever you need to change the image you just have to go to Pages and select the page and change the featured image.
Thank You
