Index: wp-content/themes/twentyeleven/style.css
===================================================================
--- wp-content/themes/twentyeleven/style.css	(revision 18110)
+++ wp-content/themes/twentyeleven/style.css	(working copy)
@@ -1504,6 +1504,7 @@
 img.size-thumbnail {
 	height: auto;
 	max-width: 100%;
+	width: auto;
 }
 img.wp-smiley {
 	margin-bottom: 0;