Index: wp-content/themes/twentytwelve/style.css
===================================================================
--- wp-content/themes/twentytwelve/style.css	(revision 21406)
+++ wp-content/themes/twentytwelve/style.css	(working copy)
@@ -317,6 +317,7 @@
 	box-shadow: 0 1px 4px rgba(0, 0, 0, 0.2);
 }
 .wp-caption {
+	max-width: 100%;
 	padding: 4px;
 }
 .wp-caption .wp-caption-text,