If you set imageClickMode="OPEN_URL" ('Main Image' options), then, when a main image in the gallery is clicked, the corresponding linkURL (entered on the gallery page's 'Images' tab) is opened in the specified linkTarget window.
This would almost certainly be the easiest way to set something like this up.
However, it would be the main image that would need to be clicked (not the thumbnail) and the linkURL web page would be opened in a new tab (rather than in a pop-up window).
If you've not got many sample videos to display, then perhaps you could set up a separate 'Basic' page and embed the videos into this page (instead of trying to incorporate them into your Juicebox gallery).
Here's a sample 'Basic' page in a demo Showkase site which embeds a couple of videos.
If you really want to have a video play in a pop-up window when a thumbnail is clicked, then try the following:
(1) Create a separate web page manually (not in Showkase) and embed your video into it by whatever method you like (HTML 5 <video> tag, YouTube embed, etc.).
(2) Upload your video's web page to your web server.
(3) Enter the URL to this web page as the corresponding image's linkURL (on the gallery page's 'Images' tab).
(4) Edit the gallery page in Showkase, click 'Source' on the editor's toolbar to enter source mode and add the following code to your web page.
<script>
function openWindow() {
window.setTimeout(function() {
var index = jb.getImageIndex();
var info = jb.getImageInfo(index);
var url = info.linkURL;
window.open(url, '_blank', "height=400,width=600");
}, 500);
}
jb.onShowThumbs = function(showing) {
if (showing) {
window.setTimeout(function() {
$('#jb-glry-id-0_thumb_0, #jb-glry-id-0_thumb_1').not('.bound').addClass('bound').click(function() {
openWindow();
});
}, 500);
}
};
jb.onThumbPageChange = function() {
window.setTimeout(function() {
$('#jb-glry-id-0_thumb_0, #jb-glry-id-0_thumb_1').not('.bound').addClass('bound').click(function() {
openWindow();
});
}, 500);
};
</script>
This example displays a pop-up window for the 1st and 2nd thumbnails in the gallery.
The id #jb-glry-id-0_thumb_0 represents the 1st image in the gallery (the internal numbering starts at 0 for the 1st image). You can change the thumbnails as required and extend this example to work for more thumbnails (each loading their own individual linkURL when clicked) if you like.
The code above uses the Juicebox-Pro API to check when a new thumbnail page is displayed (onThumbPageChange()) as the thumbnail you want to add the custom click handler to might not be visible when the gallery is initially displayed (it might be on a subsequent thumbnail page and might not initially be included in the Document Object Model). The click handler also needs to be added when the thumbnails are shown after being hidden (onShowThumbs(showing)). (The code also uses the API to fetch the image's linkURL.)
If you just had a single video per gallery, then you could hardcode the URL to the video's web page into a window.open command (and do away with the openWindow() function) but the code above will allow for multiple thumbnails to be selected (each with their own linkURL).
This is not a perfect solution by any means (a couple of short delays are required to ensure that things are in place) but it might at least point you in the right direction.