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 via FTP, on the files and folders it lists there in case chmod fails). You can disable TimThumb with the 'Use TimThumb' setting and the option 'No'.

Also read the troubleshooting guide in the documentation on what else to do, especially if you are using Hostgator!

If you are using a Better WP Security plug-in go to the Better WP Security settings, System Tweaks, Filter Suspicious Query Strings: Disable

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. Read more at: jetpack.me