Index: wp-content/themes/twentytwelve/style.css
===================================================================
--- wp-content/themes/twentytwelve/style.css	(revision 21437)
+++ wp-content/themes/twentytwelve/style.css	(working copy)
@@ -285,7 +285,6 @@
 img.header-image,
 img.wp-post-image {
 	max-width: 100%;
-	width: auto; /* Prevent stretching of full-size and large-size images with height and width attributes in IE8 */
 	height: auto; /* Make sure images with WordPress-added height and width attributes are scaled correctly */
 }