If image 'src' is '(unknown)' replace with 'missing.png'

This should work for you, if you use onError event for handling missing image src

<img src="main_img.png" onError="this.onerror=null;this.src='/images/missing.png';" />

I used this to solve a similar problem I had (using javascript)...

const images = document.querySelectorAll("img");

images.forEach((image) => {
  let imgsrc = image.getAttribute("src");
  if (imgsrc === "") {
    image.src = "/images/missing.png";
  }
});