PLP Documentation
The PLP header image will, by default, come from the collection image. This can also be overridden using the following metafields:
header.image_desktop - will override the collection image on desktop; recommended dimensions are 2500px wide by whatever height is needed
header.image_mobile - will override the collection image on mobile; recommended dimensions are 768px wide by whatever height is needed
Note that if only one metafield is specified, then the same image will be shown both on mobile and desktop; it is not necessary to specify both.