Justified Image Grid itself doesn't offer any bulk upload tools. There is a simple reason, this is not a gallery management plugin, but a display. On the other hand, it uses content sources where FTP upload in bulk is possible or can be achieved easily.
WordPress Media Library
WordPress doesn't have an out of the box solution to bulk import photos to the Media Library or detect images in a directory. However, with a good old plugin it quickly becomes child's play.
- Install the free Add From Server plugin. It's old but works well.
- Upload photos via FTP to a folder inside the /wp-content/uploads/ directory.
- Go to Media > Add From Server screen.
- Navigate to your folder and tick the checkbox next to the File column label.
- Click the Import button. Your images are now in the Media Library.
- The plugin called Media Library Assistant is recommended to categorize or tag these in bulk, so it's easier to select them for display.
It's incredibly easy.
- Create a new gallery by going to Gallery > Manage Galleries > Add new gallery button.
- Add a name such as cats then click OK button. It'll create a folder for you.
- Click Edit gallery in the notification at the top.
- Upload photos via FTP to /wp-content/gallery/cats/ folder.
- On the gallery edit screen click Scan Folder for new images. Thumbnails will be created and it's done.