Custom Image Page
A custom page can be set to a specific image:
Click on Select Image to upload an image from your system.
Bidders will see the image full screensize on their device (preserving the aspect ratio)
It is hard to give a single recommendation for resolution for these images since mobile devices have many different screen sizes.
For a portrait image a size of 1080x1920 is a good size, and rotated to landscape 1920x1080