Index: wp-includes/css/customize-controls.dev.css
===================================================================
--- wp-includes/css/customize-controls.dev.css	(revision 20913)
+++ wp-includes/css/customize-controls.dev.css	(working copy)
@@ -390,6 +390,7 @@
 
 .customize-section .customize-control-image .preview-thumbnail img {
 	display: block;
+	width: 100%;
 	max-width: 122px;
 	max-height: 98px;
 	margin: 0 auto;