If you have properly installed the plugin (and enabled it) and properly embedded the {gallery}...{/gallery} tags in some Joomla! article or K2 item (etc.) but all you see is a grid of blank/empty thumbnails, here's what's happening: Your site's /cache folder is either not writable or Joomla! (PHP to be exact) doesn't have "owner" permissions to access this folder. So the plugin cannot create (write) the gallery thumbnails and thus why you see blank/empty thumbnails.

The first case is easy to solve. Simply download the free "Admin Tools" extension from Akeeba at: goo.gl/qeBZc - Then use it to fix all file and folder permissions to what Joomla! requires to work properly. It's a matter of just clicking the "repair permissions" button in Admin Tools.

If the above doesn't fix your problem, then you are facing the second case as described above (owner permission issues). In that case you need to contact your hosting company and ask them to correct owner permissions for your Joomla! site or your entire hosting account. 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.