diff --git a/imagecache.module b/imagecache.module
index 743e90c..8bb3c2c 100644
--- a/imagecache.module
+++ b/imagecache.module
@@ -315,19 +315,22 @@ function imagecache_action_definition($action) {
 }
 
 /**
- * Return a URL that points to the location of a derivative of the
- * original image transformed with the given preset.
+ * Return a URL that points to the location of a derivative of the original
+ * image transformed with the given preset.
  *
  * Special care is taken to make this work with the possible combinations of
  * Clean URLs and public/private downloads. For example, when Clean URLs are not
  * available an URL with query should be returned, like
- * http://example.com/?q=files/imagecache/foo.jpg, so that imagecache is able
+ * http://example.com/?q=files/imagecache/foo.jpg, so that ImageCache is able
  * intercept the request for this file.
  *
- * This code is very similar to the Drupal core function file_create_url(), but
+ * This code began similarly to Drupal core's function file_create_url(), but
  * handles the case of Clean URLs and public downloads differently however.
+ * It also implements hook_file_url_alter() which was added to Drupal 7 and
+ * backported to PressFlow 6.x.
  *
  * @param $presetname
+ *   String specifying an ImageCache preset name.
  * @param $filepath
  *   String specifying the path to the image file.
  * @param $bypass_browser_cache
@@ -338,16 +341,30 @@ function imagecache_action_definition($action) {
  *   A Boolean indicating that the URL should be absolute. Defaults to TRUE.
  */
 function imagecache_create_url($presetname, $filepath, $bypass_browser_cache = FALSE, $absolute = TRUE) {
-  $path = _imagecache_strip_file_directory($filepath);
-
-  $args = array('absolute' => $absolute, 'query' => empty($bypass_browser_cache) ? NULL : time());
-  switch (variable_get('file_downloads', FILE_DOWNLOADS_PUBLIC)) {
-    case FILE_DOWNLOADS_PUBLIC:
-      $base = $absolute ? $GLOBALS['base_url'] .'/' : '';
-      return url($base . file_directory_path() ."/imagecache/$presetname/$path", $args);
-    case FILE_DOWNLOADS_PRIVATE:
-      return url("system/files/imagecache/$presetname/$path", $args);
+  $args = array('query' => empty($bypass_browser_cache) ? NULL : time());
+  $file_directory = file_directory_path();
+
+  // Determine the path of the derivative inside the files directory.
+  $derivative_path = 'imagecache/'. $presetname .'/'. _imagecache_strip_file_directory($filepath);
+
+  // Then construct a full path and see if anyone wants to alter it.
+  $altered_path = $old_path = $file_directory .'/'. $derivative_path;
+  drupal_alter('file_url', $altered_path);
+
+  // If any module has altered the path, then return the alteration...
+  if ($altered_path != $old_path) {
+    // ...but use url() so our $bypass_browser_cache parameter is honored.
+    return url($altered_path, $args);
   }
+
+  // It was unchanged so use the download method's prefix.
+  $prefix = array(
+    FILE_DOWNLOADS_PUBLIC => $file_directory,
+    FILE_DOWNLOADS_PRIVATE => 'system/files',
+  );
+  $path = $prefix[variable_get('file_downloads', FILE_DOWNLOADS_PUBLIC)] .'/'. $derivative_path;
+
+  return url($path, $args + array('absolute' => $absolute));
 }
 
 /**