When viewing your listing details page, by default the photos viewer/slideshow will automatically crop and size to fill the available space with the image. For vertical images, this can cause most the image to not appear in the initial preview, unless the user was to view the large version of the photo.
If you prefer the photo viewed not automatically crop and zoom in on the photo, you can turn this setting off.
From your control panel do the following:
Click on My Listings.
Click on Featured Listing Settings.
Find the setting "Disable Automatic Crop & Fit Photos".
Select the option to disable the setting.
Click to Save Changes.
Now when you view your listing photos, the entire image should display, including vertical images.