Fix minor bugs

lib/logic/Task.js

@@ -15,7 +15,7 @@ const schema = require("@colyseus/schema"),
     duration: 15000,
   },
   {
-    imageUrl: "images/Bruegel-Childrens_Games.jpg.jpg",
+    imageUrl: "images/Bruegel-Childrens_Games.jpg",
     taskDescription: "Identifizieren Sie unterschiedliche Spiele.",
     taskSource: "Pieter Bruegel the Elder, Children's Bruegel-Childrens_Games (Oil on oak wood, 1560), Kunsthistorisches Museum, Vienna",
     duration: 15000,

www/resources/js/viewer/ImageViewer.js

@@ -60,6 +60,7 @@ class ImageViewer {
       image = new Image();
     image.src = imageUrl;
     image.onload = function() {
+      that.backgroundContext.clearRect(0, 0, screen.width, screen.height);
       that.backgroundContext.drawImage(image, 0, 0);
     };
   }