The point where Justified Image Grid is open for extensibility is the RSS feeds source. They are basically just XML documents. It's possible to craft such document manually or using PHP. Stay tuned for more detailed tutorials on these subjects.
With a relatively simple markup, you'd add source URL and title/description/date to include any external photo in a list. You could even make the photos link to somewhere specific. After that, JIG would take them and display a beautiful gallery. It'll determine the size of each photo then fetch and process corresponding metadata. The result can open in the lightbox or have clicks directed at your custom links.
<?xml version="1.0" ?>
<title>Title of the test feed</title>
<description>To get external images to JIG</description>
<title>The first photo</title>
<img src="https://dummyimage.com/600x600/3e2723/ff5722/&text=Dummy photo" />
this is just an example with a link and a date
<pubDate>Thu, 25 Jun 2015 15:16:42 GMT</pubDate>
<title>The second one</title>
<img src="https://dummyimage.com/1280x720/795548/ffc107" />
using some dummy images, no date, no link
For a more automatic way, one would use PHP to scan an FTP folder and generate an RSS feed of it. Maybe such script already exists, or else you could hire a freelancer to create it for you according to your specific needs. While you may not require metadata such as title and description info, it's always a nice addition to a gallery. Since the filenames can be less meaningful using letters and numbers, it's worth noting that PHP could read IPTC metadata from the files for the "ultimate FTP to RSS feed script".