as you know all pages have preview images. I would like to ask that is there any way to set up a default one which means if a preview image not uploaded use this?
If I have understood this correctly, you should be able to choose a featured image for a website/landing page. To find this, click on the Settings tab within the website page and if you scroll down, you should see 'Featured Image'.
Let me know if this isn't helpful and I can help troubleshoot futher.