Replace images that don't work

[skittery]

Strange question - I have a list of images that a few of them sometimes are broken links (long story).
Before I started using lazy load, I had a script that replaced them with a fallback general image if the image was a broken link (using !isImageOk).
How can I add an exception that when adding an image when lazyloading, it will check if the image is ok and if not it will add a fallback image?