How to access images on server for CiviMail
You may want to check Firewall or the settings in WPSecurity (plugin) if you have that installed.  I ran into this problem and it was an option in WPSecurity for "Bad Query Strings" which blocked access to thumbnails in KCFinder. 

According to the plugin... Deny Bad Query Strings


  This feature will write rules in your .htaccess file to prevent 
  malicious string attacks on your site using XSS. NOTE: Some of these
  strings might be used for plugins or themes and hence this might break
  some functionality. You are therefore strongly advised to take a
  backup of your active .htaccess file before applying this feature.