updated screenshot

Signed-off-by: Jonas Sulzer <jonas@violoncello.ch>

composer.json

@@ -14,7 +14,7 @@
         }
     ],
     "extra": {
-        "image": "image.jpg"
+        "image": "image.png"
     },
     "archive": {
         "exclude": ["!/app", "!/css", "/app/assets"]