Header set Cache-Control: 'public'

RewriteEngine On

RewriteCond %{REQUEST_URI}  thumbs/   
RewriteCond %{REQUEST_URI}  !kln.jpg$   
RewriteCond %{REQUEST_URI}  !grt.jpg$   
RewriteRule thumbs/(.*)\.jpg$   /thumbs/$1_kln.jpg [L]

RewriteCond %{REQUEST_URI}  large/   
RewriteCond %{REQUEST_URI}  !kln.jpg$   
RewriteCond %{REQUEST_URI}  !grt.jpg$   
RewriteRule large/(.*)\.jpg$   /large/$1_grt.jpg [L]