Index: src/wp-admin/css/customize-controls.css
===================================================================
--- src/wp-admin/css/customize-controls.css	(revision 37955)
+++ src/wp-admin/css/customize-controls.css	(working copy)
@@ -939,7 +939,7 @@
 
 .customize-control .attachment-media-view .thumbnail-image img,
 .customize-control-header img {
-	width: 100%;
+	max-width: 100%;
 }
 
 .customize-control .attachment-media-view .remove-button,