Re-added PHP register block for front-end render;

Added example image

src/blocks/activity/index.js

@@ -5,6 +5,6 @@ registerBlockType( 'wp-strava/activity', {
     title: 'Strava Activity',
     icon: 'chart-line',
     category: 'widgets',
-    edit: () => <img src={ActivityImage} />,
+    edit: () => <img className="wp-strava-img" src={ActivityImage} />,
     save: () => <div>Hola, mundo!</div>,
 } );