Save visible image tags to app data directory then update HTML with ms-appdata uris and save HTML to app data directory.

saveCanvasBitmapsToFiles.js

    function SaveImage(strm, localfolder, filename) {
        return localfolder.createFileAsync(filename, Windows.Storage.CreationCollisionOption.replaceExisting).then(function (file) {
            return file.openAsync(Windows.Storage.FileAccessMode.readWrite);
        }).then(function (ras) {
            var os = ras.getOutputStreamAt(0),
                is = strm.getInputStreamAt(0);

            return Windows.Storage.Streams.RandomAccessStream.copyAndCloseAsync(is, os);
        });
    }

    function SaveSplash() {

        window.msWriteProfilerMark("start-render");

        var body = document.body,
            appdata = Windows.Storage.ApplicationData.current,
            localfolder = appdata.localFolder,
            canvas = document.createElement("canvas"),
            ctx = canvas.getContext("2d"),
            imgs = document.body.getElementsByTagName("img"),
            size = imgs.length,
            promises = [],
            clientWidth = body.clientWidth,
            clientHeight = body.clientHeight,
            i = 0, box, img, blob, strm, dataReader, cnt = 1, filename;

        for (var i = 0; i < size; i++) {
            img = imgs.item(i);

            try {
                box = img.getBoundingClientRect();
            } catch (e) {
            }

            if (!!box &&
                box.left <= clientWidth &&
                box.top <= clientHeight &&
                box.left + box.width >= 0 &&
                box.top + box.height >= 0) {

                canvas.width = box.width;
                canvas.height = box.height;
                ctx.drawImage(img, 0, 0, box.width, box.height);

                blob = canvas.msToBlob();
                strm = blob.msDetachStream();

                filename = "splash" + cnt + ".png";
                cnt += 1;
                img.src = "ms-appdata:///local/" + filename;

                promises.push(SaveImage(strm, localfolder, filename));
            } else {
                img.src = "";
            }
        }

        return WinJS.Promise.join(promises).then(function () {
            window.msWriteProfilerMark("start-encode");

            blob = body.innerHTML;

            window.msWriteProfilerMark("start-write");

            return localfolder.createFileAsync("splash.htm", Windows.Storage.CreationCollisionOption.replaceExisting);
        }).then(function (file) {
            return Windows.Storage.FileIO.writeTextAsync(file, blob);
        });
    }

// ...

    app.oncheckpoint = function (args) {

        window.msWriteProfilerMark("start-suspend");

        args.setPromise(SaveSplash().then(function () {
            window.msWriteProfilerMark("stop-suspend");
        }));
    };