This is most likely a TimThumb permissions error.
Go to the Justified Image Grid settings, TimThumb & CDN tab. Click check permissions then click 0755 or 0777 to see if that works (or do it manually on the files and folders it lists there in case chmod fails).
Also read the troubleshooting guide in the documentation on what else to do.
Tip: Install the official WP plugin ‘Jetpack’ by Automattic and enable ‘Photon’. Jetpack enables you to connect your blog to a WordPress.com account to use the powerful features normally only available to WordPress.com users. It’s an excellent TimThumb alternative and will make your images load faster. Note that you won’t be able to use special effects due to cross-domain security limitations. It’ll also make Justified Image Grid high resolution display or Retina ready by devicepx.js! Read more at: jetpack.me
Another possibility is that you are using W3 Total Cache to minify JS – turn that feature off.