Original code author: ExpertSystem
http://stackoverflow.com/a/19831667/4101334

/* Download an img */
function download(img) {
    var link = document.createElement("a");
    link.href = img.href;
    link.download = img.title;
    link.style.display = "none";
    var evt = new MouseEvent("click", {
        "view": window,
        "bubbles": true,
        "cancelable": true
    });

    document.body.appendChild(link);
    link.dispatchEvent(evt);
    document.body.removeChild(link);
    console.log("Downloading...");
}

/* Download all images in 'imgs'.
 * Optionaly filter them by extension (e.g. "jpg") and/or
 * download the 'limit' first only  */
function downloadAll(imgs, ext, limit) {
    /* If specified, filter images by extension */
    if (ext) {
        ext = "." + ext;
        imgs = [].slice.call(imgs).filter(function(img) {
            var src = img.src;
            return (src && (src.indexOf(ext, src.length - ext.length) !== -1));
        });
    }

    /* Determine the number of images to download */
    limit = (limit && (0 <= limit) && (limit <= imgs.length))
            ? limit : imgs.length;

    /* (Try to) download the images */
    for (var i = 0; i < limit; i++) {
        var img = imgs[i];
        console.log("WEBM: " + img.href + " (", img, ")");
        download(img);
    }
}


/* Callback for button's "click" event*/
// Look only inside .desktop class elements
// for href that starts with /b/src/ and ends with .webm
function doit() {
    var imgs = $('.desktop[href^="/b/src/"][href$=".webm"]');
    downloadAll(imgs, "", -1);
}

/* Callback for button's "click" event */
// Look only inside .desktop class elements
// for href that starts with /b/src/ and not ends with .webm
function doit2() {
    var imgs = $('.desktop[href^="/b/src/"]').not('.desktop[href$=".webm"]');
    downloadAll(imgs, "", -1);
}

/* Create and add a "download" button on the top, left corner */
function addDownloadBtn() {
    var btn = document.createElement("button");
    btn.innerText = "Download all шебм";
    btn.addEventListener("click", doit);
    btn.style.position = "fixed";
    btn.style.top = btn.style.left = "0px";
    document.body.appendChild(btn);

    var btn2 = document.createElement("button");
    btn2.innerText = "Download all images";
    btn2.addEventListener("click", doit2);
    btn2.style.position = "fixed";
    btn2.style.top = "0px";
    btn2.style.left = "150px";
    document.body.appendChild(btn2);
}
addDownloadBtn();