always show the image in the middle of the screen, regardless of scrolling

apps/files_imageviewer/css/lightbox.css

 #lightbox_overlay{
-	position:absolute;
+	position:fixed;
 	height:100%;
 	width:100%;
 	top:0px;
@@ -11,7 +11,7 @@
 }
 
 #lightbox{
-	position:absolute;
+	position:fixed;
 	max-height:90%;
 	max-width:90%;
 	top:10px;