All of the images have failed to load

If you received this dreaded message, please don't worry. There is a solution 100% of the time. There could be a number of reasons why your thumbnails won't load. It's either a server setup related issue (file and folder permissions), your security plugins getting proactive, malformed Content Delivery Network URL or something else, in which case you could just ask for support. Here are the steps used to find the problem:

1. Right click on your page and choose Inspect. (Chrome, Firefox)
2. Switch to the Network tab.
3. Reload the page.
4. Find a line that begins with timthumb and apparently fails.
5. Right click on that line and open it in new tab.
6. You'll either get a TimThumb HTML page explaining the problem, or 403, 404 or 500 error (status code).

Go to the JIG Plugin Settings > TimThumb & CDN tab. Set Use TimThumb to No. Below that, you can pick a WordPress-generated thumbnail size (if a small one exists) with the Base thumbnail on an image size setting.

This combo turns off Retina Ready and much of the nice thumbnail features JIG is known for. See why it's advised to fix the problem rather than the symptom in the Why does JIG use TimThumb? FAQ article.

• iThemes Security (formerly Better WP Security) settings > System Tweaks > Configure Settings > Disable PHP in Plugins – Off. Also, System Tweaks > Filter Suspicious Query Strings – Disable.
• Sucuri Security plugin > 1-Click Hardening – Do not use it.
• WP All in ONE Security:  WP Security > Firewall > Additional Firewall Rules (tabs at top of page) > Bad Query Strings – Untick.
  AND/OR WP Security > Firewall > 6G Blacklist Firewall Rules (tabs at top of page) > Enable 6G Firewall Protection – Untick.
• Hide My WP settings > General Settings > Exclude Files – add this:
  
  wp-content/plugins/justified-image-grid/timthumb.php

  1	wp-content/plugins/justified-image-grid/timthumb.php

  Read more about this at the Hide My WP FAQ.

• Are you using Hostgator? Simply contact their support via phone or live chat to report mod security blockage. Send them the faulty link to a thumbnail, that you opened on a new tab in step 5. 
• If you are not on Hostgator but getting 403 errors then try adding these lines to your .htaccess file in the WordPress installation folder. Revert the modification if it has no effect and contact the hosting's support.
  
  <IfModule mod_security.c> SecFilterEngine Off SecFilterScanPOST Off </IfModule>

  1 2 3 4

  <IfModule mod_security.c>   SecFilterEngine Off   SecFilterScanPOST Off </IfModule>

• Are you on Bluehost's Optimized Hosting for WordPress? Make sure you turn off Varnish caching. It empties query strings.
• Try switching JIG settings > TimThumb & CDN > Thumbnail filename to the Extensionless version option (Bluehost too).
• Open your .htaccess file and ensure you do not have this line in it:
  
  php_value mbstring.func_overload 2

  1	php_value mbstring.func_overload 2

• Or something like this:
  
  RewriteRule wp-content/plugins/(.*\.php)$ - [R=404,L]

  1	RewriteRule wp-content/plugins/(.*\.php)$ - [R=404,L]

• Make sure you have GD library. A possible error is "GD Library Error: imagecreatetruecolor does not exist - please contact your webhost and ask them to install the GD library".

• Go to the JIG settings > TimThumb & CDN tab. Click check permissions and look for a green image. Follow the instructions there. You can try changing the permission using the tool or if it fails, do it manually via FTP (on the files and folders the tool mentions).

• Make sure you use the JIG settings > TimThumb & CDN tab > Replace site's hostname with setting. It must not start with http, contain any slashes or spaces.
• To temporarily lift the thumbnails off of the CDN, use the Custom TimThumb path setting with an URL like this:
  
  http://exampledomain.com/wp-content/plugins/justified-image-grid/timthumb.php

  1	http://exampledomain.com/wp-content/plugins/justified-image-grid/timthumb.php

  Note that your directory structure may be different.
• If you feel that your CDN is not able to provide thumbnails, contact the support of either the CDN or JIG for more information. Usually all it takes is to change a couple of settings on the CDN.
• If you use a WordPress plugin to connect with your CDN, let that be W3 Total Cache.

If all options above are failing, just install the Jetpack plugin and enable the Photon module. It's a thumbnail creation alternative and will make your images load faster as it's like a free CDN. Note that you won't be able to use special effects due to cross-domain security limitations.

• Read more at jetpack.me
• About Content Delivery Networks, including Photon