Index: includes/DefaultSettings.php
===================================================================
--- includes/DefaultSettings.php	(revision 83265)
+++ includes/DefaultSettings.php	(working copy)
@@ -4619,6 +4619,12 @@
 );
 
 /**
+ * Extension of "thumbnails" that are very expensive to regenerate and should be 
+ * excluded from normal action=purge thumbnail removal. 
+ */
+$wgExcludeFromThumbnailPurge = array();
+
+/**
  * Additional functions to be performed with updateSpecialPages.
  * Expensive Querypages are already updated.
  */
Index: includes/filerepo/LocalFile.php
===================================================================
--- includes/filerepo/LocalFile.php	(revision 83265)
+++ includes/filerepo/LocalFile.php	(working copy)
@@ -659,7 +659,7 @@
 	 * Delete cached transformed files
 	 */
 	function purgeThumbnails() {
-		global $wgUseSquid;
+		global $wgUseSquid, $wgExcludeFromThumbnailPurge;
 
 		// Delete thumbnails
 		$files = $this->getThumbnails();
@@ -667,6 +667,12 @@
 		$urls = array();
 
 		foreach ( $files as $file ) {
+			// Only remove files not in the $wgExcludeFromThumbnailPurge configuration variable
+			$ext = pathinfo("$dir/$file", PATHINFO_EXTENSION);
+			if( in_array( $ext, $wgExcludeFromThumbnailPurge ) ){
+				continue;
+			}
+			
 			# Check that the base file name is part of the thumb name
 			# This is a basic sanity check to avoid erasing unrelated directories
 			if ( strpos( $file, $this->getName() ) !== false ) {