How to Check if An Image Exists in a Document and Generate One if Not

I am using this plugin for generating images, but i want to automate it, lets say no image has been added, this plugin should automatically generate an image for the document

you'd probably want to use document actions, i.e. when a document is published, check if there's an image and create one if not.

https://www.sanity.io/docs/document-actions