use :extend in thumbnail Less

dist/css/bootstrap.css

@@ -307,7 +307,9 @@ figure {
 img {
   vertical-align: middle;
 }
-.img-responsive {
+.img-responsive,
+.thumbnail > img,
+.thumbnail a > img {
   display: block;
   max-width: 100%;
   height: auto;
@@ -4401,9 +4403,6 @@ a.list-group-item.active > .badge,
 }
 .thumbnail > img,
 .thumbnail a > img {
-  display: block;
-  max-width: 100%;
-  height: auto;
   margin-right: auto;
   margin-left: auto;
 }

less/thumbnails.less

@@ -16,7 +16,7 @@
 
   > img,
   a > img {
-    .img-responsive();
+    &:extend(.img-responsive);
     margin-left: auto;
     margin-right: auto;
   }