Unfortunately, Google (who acquired Picasa) do not seem to allow access to new Google Photo Albums via the Picasa Web API although existing albums created prior to the acquisition can still be displayed via WP-Juicebox (as long as they have not been modified recently).

This is by design. When the 'Open Image' button is clicked for a Flickr gallery, the image's Flickr page is opened.
There is no way to change this behavior using any available configuration options. The code which handles this is buried deep within the 'juicebox.js' file which is obfuscated and cannot be modified.

All I can suggest at the moment is to perhaps suggest this alternative action for the 'Open Image' button in the Feature Requests forum thread.
This keeps all the ideas together and ensures that they are not overlooked by the developers.
I do not know the likelihood of any suggestions being implemented but this is certainly the best place for all ideas.
Thank you.

Thank you for posting your idea in the Feature Requests thread.

I've found a workaround for this which you might like to try.
It's rather complex and involves overriding Juicebox-Pro's own click handler for the 'Open Image' button.
It uses the Juicebox-Pro API (specifically the onInitComplete event) to ensure that the 'Open Image' button is present in the DOM (Document Object Model) before any actions are performed on it.
It also uses JavaScript to extract the current image's URL (from a dynamically generated container with a specific CSS class name) before opening it in a new tab/window.

<!--START JUICEBOX EMBED-->
<script src="jbcore/juicebox.js"></script>
<script>
    var jb = new juicebox({
        containerId: "juicebox-container",
        useFlickr: "TRUE",
        flickrUserName: "your_flickr_username"
    });
    jb.onInitComplete = function() {
        $('#juicebox-container .jb-bb-btn-open-url').off('click');
        $('#juicebox-container .jb-bb-btn-open-url').click(function() {
            var index = jb.getImageIndex() - 1;
            var element = '#juicebox-container .jb-dt-main-image-' + index;
            var src = $(element).find('img').first().attr('src');
            window.open(src, '_blank');
        });
    };
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

I hope this is a suitable workaround for you.

If you want to incorporate it into a WP-Juicebox gallery, then add the following code to the body of your WordPress page or post directly below your Juicebox gallery shortcode. (Make sure that your editor is in 'Text' mode rather than 'Visual' mode.)

<script type="text/javascript">
    jb_[gallery_id].onInitComplete = function() {
        jQuery('#juicebox-container-[gallery_id] .jb-bb-btn-open-url').off('click');
        jQuery('#juicebox-container-[gallery_id] .jb-bb-btn-open-url').click(function() {
            var index = jb_[gallery_id].getImageIndex() - 1;
            var element = '#juicebox-container-[gallery_id] .jb-dt-main-image-' + index;
            var src = jQuery(element).find('img').first().attr('src');
            window.open(src, '_blank');
        });
    };
</script>

Replace the five instances of [gallery_id] in the code above with the actual Gallery Id which you can find in the Juicebox gallery shortcode.
For example, if your Gallery Id is 7, then your code should look like this:

<script type="text/javascript">
    jb_7.onInitComplete = function() {
        jQuery('#juicebox-container-7 .jb-bb-btn-open-url').off('click');
        jQuery('#juicebox-container-7 .jb-bb-btn-open-url').click(function() {
            var index = jb_7.getImageIndex() - 1;
            var element = '#juicebox-container-7 .jb-dt-main-image-' + index;
            var src = jQuery(element).find('img').first().attr('src');
            window.open(src, '_blank');
        });
    };
</script>

If you have trouble with WordPress adding <br> and <p> tags throughout the code, install the Raw HTML plugin and wrap the code in [raw] ... [/raw] tags.

Please note that Juicebox-Pro was not designed with this functionality in mind (user overrides for Button Bar buttons) and whilst you are free to use such modifications, there are not officially supported (and you may run into unforeseen problems that might have to be tackled along the way).