diff --git a/preview.go b/preview.go
index 61cee4f..7cad364 100644
--- a/preview.go
+++ b/preview.go
@@ -77,6 +77,7 @@ html, body {
 .thumbnail {
 	line-height: 0;
 	width: 340px;
+	height: 340px;
 	text-align: center;
 	position: relative;
 }