Full-width parallax banner strips show only a horizontal slice of your photo, roughly the middle third of a landscape image. If you use a vertical (portrait) photo, only a thin band of it appears in these spots.
What to use. Choose photos that are wider than they are tall, at least 1920 by 1080 pixels. Keep the subject centered in the frame, with nothing important near the top or bottom edges.
Sending the file. Send the photo at full resolution. Do not send a screenshot or a document export, because these can cap images at around 2048 pixels.