CAT_VALIDATION_ERROR_ADDITIONAL_IMAGES_LINK_INVALID is raised when an entry in additional_image_link cannot be parsed as a valid URL. Each supplementary image needs its own absolute, publicly reachable URL.
Snap Catalog Manager requires product images that are publicly reachable, sized within the platform's bounds, in supported formats (JPEG/PNG typically), and free of overlays, watermarks, retailer logos, or text callouts. This issue trips when one of those constraints is broken.