Custom Fields:
You can use custom fields or post meta to add an additional image URL for the main page preview. Many themes and plugins support custom fields, and you can display the custom field value in the template where you want to show the main page preview.
Custom Post Types:
Create a custom post type specifically for the main page preview, and associate it with the actual post. You can then use a custom template or code to display the custom post type’s featured image on the main page.