Your homepage comes with default images defined by your theme. To replace them with photos of your own, locate the default versions in your Admin Area by opening your Store Editor app and accessing the Content section. From the Home menu, select the relevant heading(s) that may contain the image(s) in question (options differ by theme).
As an example, if you wanted to replace the images in the "Popular Categories" homepage section of the Montreal theme, select Homepage Content and press the Featured Content heading of your choice.
In all cases, access the photo drop zone by hovering over the picture in the Image section and pressing the pencil icon. From here, you can upload a new photo to the Image Gallery or select an existing one. For more information, see "How to Manage and Edit Images".