diff --git a/lib/filelib.php b/lib/filelib.php
index 84068078dcc..b5767e75ba9 100644
--- a/lib/filelib.php
+++ b/lib/filelib.php
@@ -2188,11 +2188,11 @@ function send_temp_file($path, $filename, $pathisstring=false) {
 
     header('Content-Disposition: attachment; filename="'.$filename.'"');
     if (strpos($CFG->wwwroot, 'https://') === 0) { //https sites - watch out for IE! KB812935 and KB316431
-        header('Cache-Control: private, max-age=10');
+        header('Cache-Control: private, max-age=10, no-transform');
         header('Expires: '. gmdate('D, d M Y H:i:s', 0) .' GMT');
         header('Pragma: ');
     } else { //normal http - prevent caching at all cost
-        header('Cache-Control: private, must-revalidate, pre-check=0, post-check=0, max-age=0');
+        header('Cache-Control: private, must-revalidate, pre-check=0, post-check=0, max-age=0, no-transform');
         header('Expires: '. gmdate('D, d M Y H:i:s', 0) .' GMT');
         header('Pragma: no-cache');
     }
@@ -2274,18 +2274,18 @@ function send_file($path, $filename, $lifetime = null , $filter=0, $pathisstring
             $private = ' private,';
         }
         $nobyteserving = false;
-        header('Cache-Control:'.$private.' max-age='.$lifetime);
+        header('Cache-Control:'.$private.' max-age='.$lifetime.', no-transform');
         header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
         header('Pragma: ');
 
     } else { // Do not cache files in proxies and browsers
         $nobyteserving = true;
         if (strpos($CFG->wwwroot, 'https://') === 0) { //https sites - watch out for IE! KB812935 and KB316431
-            header('Cache-Control: private, max-age=10');
+            header('Cache-Control: private, max-age=10, no-transform');
             header('Expires: '. gmdate('D, d M Y H:i:s', 0) .' GMT');
             header('Pragma: ');
         } else { //normal http - prevent caching at all cost
-            header('Cache-Control: private, must-revalidate, pre-check=0, post-check=0, max-age=0');
+            header('Cache-Control: private, must-revalidate, pre-check=0, post-check=0, max-age=0, no-transform');
             header('Expires: '. gmdate('D, d M Y H:i:s', 0) .' GMT');
             header('Pragma: no-cache');
         }
@@ -2445,17 +2445,17 @@ function send_stored_file($stored_file, $lifetime=null, $filter=0, $forcedownloa
         if (isloggedin() and !isguestuser()) {
             $private = ' private,';
         }
-        header('Cache-Control:'.$private.' max-age='.$lifetime);
+        header('Cache-Control:'.$private.' max-age='.$lifetime.', no-transform');
         header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
         header('Pragma: ');
 
     } else { // Do not cache files in proxies and browsers
         if (strpos($CFG->wwwroot, 'https://') === 0) { //https sites - watch out for IE! KB812935 and KB316431
-            header('Cache-Control: private, max-age=10');
+            header('Cache-Control: private, max-age=10, no-transform');
             header('Expires: '. gmdate('D, d M Y H:i:s', 0) .' GMT');
             header('Pragma: ');
         } else { //normal http - prevent caching at all cost
-            header('Cache-Control: private, must-revalidate, pre-check=0, post-check=0, max-age=0');
+            header('Cache-Control: private, must-revalidate, pre-check=0, post-check=0, max-age=0, no-transform');
             header('Expires: '. gmdate('D, d M Y H:i:s', 0) .' GMT');
             header('Pragma: no-cache');
         }
diff --git a/lib/tests/filelib_test.php b/lib/tests/filelib_test.php
index 657a50e49d1..f5ce378d8f2 100644
--- a/lib/tests/filelib_test.php
+++ b/lib/tests/filelib_test.php
@@ -677,7 +677,7 @@ Connection: close
 HTTP/1.0 200 OK
 Server: Apache
 X-Lb-Nocache: true
-Cache-Control: private, max-age=15
+Cache-Control: private, max-age=15, no-transform
 ETag: "4d69af5d8ba873ea9192c489e151bd7b"
 Content-Type: text/html
 Date: Thu, 08 Dec 2011 14:44:53 GMT
@@ -693,7 +693,7 @@ EOF;
 HTTP/1.0 200 OK
 Server: Apache
 X-Lb-Nocache: true
-Cache-Control: private, max-age=15
+Cache-Control: private, max-age=15, no-transform
 ETag: "4d69af5d8ba873ea9192c489e151bd7b"
 Content-Type: text/html
 Date: Thu, 08 Dec 2011 14:44:53 GMT
diff --git a/lib/weblib.php b/lib/weblib.php
index ef395228b57..a5780b0ece8 100644
--- a/lib/weblib.php
+++ b/lib/weblib.php
@@ -2053,13 +2053,13 @@ function send_headers($contenttype, $cacheable = true) {
 
     if ($cacheable) {
         // Allow caching on "back" (but not on normal clicks).
-        @header('Cache-Control: private, pre-check=0, post-check=0, max-age=0');
+        @header('Cache-Control: private, pre-check=0, post-check=0, max-age=0, no-transform');
         @header('Pragma: no-cache');
         @header('Expires: ');
     } else {
         // Do everything we can to always prevent clients and proxies caching.
         @header('Cache-Control: no-store, no-cache, must-revalidate');
-        @header('Cache-Control: post-check=0, pre-check=0', false);
+        @header('Cache-Control: post-check=0, pre-check=0, no-transform', false);
         @header('Pragma: no-cache');
         @header('Expires: Mon, 20 Aug 1969 09:23:00 GMT');
         @header('Last-Modified: ' . gmdate('D, d M Y H:i:s') . ' GMT');
diff --git a/theme/image.php b/theme/image.php
index e3839b28b21..99af6c3433e 100644
--- a/theme/image.php
+++ b/theme/image.php
@@ -111,7 +111,7 @@ if ($rev > 0) {
             $mimetype = get_contenttype_from_ext($ext);
             header('HTTP/1.1 304 Not Modified');
             header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
-            header('Cache-Control: public, max-age='.$lifetime);
+            header('Cache-Control: public, max-age='.$lifetime.', no-transform');
             header('Content-Type: '.$mimetype);
             header('Etag: "'.$etag.'"');
             die;
@@ -217,7 +217,7 @@ function send_cached_image($imagepath, $etag) {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($imagepath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Pragma: ');
-    header('Cache-Control: public, max-age='.$lifetime);
+    header('Cache-Control: public, max-age='.$lifetime.', no-transform');
     header('Accept-Ranges: none');
     header('Content-Type: '.$mimetype);
     header('Content-Length: '.filesize($imagepath));
diff --git a/theme/yui_image.php b/theme/yui_image.php
index ee89461eb46..5a8818eb48d 100644
--- a/theme/yui_image.php
+++ b/theme/yui_image.php
@@ -98,7 +98,7 @@ if (strpos($path, '/-1/') === false and (!empty($_SERVER['HTTP_IF_NONE_MATCH'])
     header('HTTP/1.1 304 Not Modified');
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($imagepath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
-    header('Cache-Control: public, max-age='.$lifetime);
+    header('Cache-Control: public, max-age='.$lifetime.', no-transform');
     header('Content-Type: '.$mimetype);
     header('Etag: "'.$etag.'"');
     die;
@@ -117,7 +117,7 @@ function yui_image_cached($imagepath, $imagename, $mimetype, $etag) {
     header('Last-Modified: '. gmdate('D, d M Y H:i:s', filemtime($imagepath)) .' GMT');
     header('Expires: '. gmdate('D, d M Y H:i:s', time() + $lifetime) .' GMT');
     header('Pragma: ');
-    header('Cache-Control: public, max-age=315360000');
+    header('Cache-Control: public, max-age=315360000, no-transform');
     header('Accept-Ranges: none');
     header('Content-Type: '.$mimetype);
     header('Content-Length: '.filesize($imagepath));