PROBLEM
Gallery thumbnails don't display at all in my site! I only see blank thumbnails (frames), but if I click on them, I can see the original image in the popup.


SOLUTION
The problem is clearly file/owner permission related. Here's why...

The plugin operates simply. It reads the image folder you "instruct" it to read when you enter the {gallery}...{/gallery} plugin tags. It then creates smaller images from the original ones (the thumbnails) and copies them to Joomla! /cache folder. If for some reason Joomla!'s /cache folder is not "writable", then the plugin will not be able to operate properly, as it will not be able to "write" the thumbnails it creates into the Joomla! /cache folder.

When we say that the Joomla! /cache folder is not "writable", that means 2 things:

1. The "/cache" folder has wrong folder access permissions preventing any Joomla! extension writing cache files into it. In that case, you can download the free "Admin Tools" extension from Akeeba at goo.gl/qeBZc and use the "Repair Permissions" operation to repair all your Joomla! site's file and folder permissions to 644 and 755 respectively. These are the proper permissions for files and folders in UNIX/Linux based servers.

2. The "/cache" folder has wrong "owner" permissions. Wrong "owner" permissions may incur for various reasons. E.g. you migrated your site to your new host and the old file/folder "owner" was maintained in some folders, including your "/cache" folder. What you should do, or ask your hosting company to do is:
a) connect to your site via ssh as your username (or possibly as root user)
b) navigate via ssh to your joomla site's folder
c) execute this in a shell prompt: $ chown yourusername:yourusername * -Rfv
This last command will correct all "owner" permissions in your joomla site. If you have ssh access to your server, you may not be able to correct the permissions on your own. In that case, your hosting company needs to login as the server's "root" user and correct the owner permissions.

If your hosting company refuses to do that, know that Joomla! won't operate properly on your server (updates, new extensions, file uploads etc.). In that case, you should strongly consider changing hosting companies. We recommend the following hosting companies:

forum.joomlaworks.net/forum-updates-other-resources/recommended-hosting-companies-%28in-case-you%27re-frustrated-with-your-current-one!%29/

All of them are known to have proper shared hosting server setups that are ideal to run Joomla! on.


IN CONCLUSION
This is not just an issue with this specific extension. If you don't solve file/folder permissions in your Joomla! site, then Joomla! will simply fail to function properly.