Here's what the /check endpoint returns:

{
    "recommendations": [
        {
            "id": null,
            "title": "Login • Instagram",
            "description": null,
            "excerpt": "Welcome back to Instagram. Sign in to check out what your friends, family & interests have been capturing & sharing around the world.",
            "featured_image": "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAKAAAADTbI/v/KBhsCbrDBhoAbbAi4wQYbAm6wIeAGG2wIuMGGgBtssCHgBhsCbrDB/wlKajuIhIz6AQAAAABJRU5ErkJggg==",  (Truncated before posting issue)
            "favicon": "https://static.cdninstagram.com/rsrc.php/v3/yG/r/De-Dwpd5CHc.png",
            "url": "https://www.instagram.com/yomikestevens/", 
            "one_click_subscribe": false,
            "created_at": null,
            "updated_at": null
        }
    ]
}
===================================

And here's the 422 response on trying to save the recommendation with the (incorrect) check response above.  

{
    "errors": [
        {
            "message": "Validation error, cannot save recommendation.",
            "context": "recommendations.0.featured_image must be a valid URL",
            "type": "ValidationError",
            "details": null,
            "property": null,
            "help": null,
            "code": null,
            "id": "f6d58dd1-d573-11ee-9d12-7d870f824854",
            "ghostErrorCode": null
        }
    ]
}