Index: modules/image/image.module
===================================================================
--- modules/image/image.module	(revision 2555)
+++ modules/image/image.module	(working copy)
@@ -657,6 +657,12 @@
   $image_uri = $scheme . '://' . $target;
   $derivative_uri = image_style_path($style['name'], $image_uri);
 
+  // Confirm that the original source image exists before trying to process it.
+  if (!is_file($image_uri)) {
+    drupal_not_found();
+    drupal_exit();
+  }
+
   // If using the private scheme, let other modules provide headers and
   // control access to the file.
   if ($scheme == 'private') {