See alt descriptions js console snippet

show-alts.js

// Paste the following into your js console while viewing images, e.g. on your facebook feed.
// It will show the "alt" attributes that facebook assigned your images.
// It's an interesting peek into how facebook analyzes your images.

setInterval(function() {
    var imgs = document.querySelectorAll("img");
    for (var i = 0; i < imgs.length; i++) {
        var alt = imgs[i].alt;
        if (alt && !imgs[i].getAttribute("alted")) {
            var div = document.createElement("div");
            div.append(document.createTextNode(alt));
            div.setAttribute("style", "position: absolute; top: 0; left: 0; z-index: 100; text-shadow: 0 0 4px #000; color: #fff;font-weight: bold;");
            imgs[i].parentElement.prepend(div);
            imgs[i].setAttribute("alted", "1");
        }
    }
}, 1000);