random image for post if no image exists

would like: if no image has been put in a post (new or existing), a random image would be pulled from media library and inserted in post. Want every post to have images: some predefined by me when creating post but also posts where i did not have/make time to enter image or where a random image from library would be fine.

have looked at several plugins…but would like recommendations

OR

maybe it could be done with simple code?

thanks!