update sample certificate images and fix a couple other onboarding assets to jive with new color scheme

challenges/02-data-visualization-certification/data-visualization-certificate.json

@@ -8,7 +8,7 @@
       "title": "Claim Your Data Visualization Certificate",
       "description": [
         [
-          "http://i.imgur.com/syJxavV.jpg",
+          "http://i.imgur.com/Et3iD74.jpg",
           "An image of our Data Visualization Certificate",
           "This challenge will give you your verified Data Visualization Certificate. Before we issue your certificate, we must verify that you have completed all of our basic and intermediate Bonfires, and all our basic and intermediate Ziplines. You must also accept our Academic Honesty Pledge. Click the button below to start this process.",
           ""
@@ -95,7 +95,7 @@
       "nameEs": "Reclama tu certificado de desarrollo Front End",
       "descriptionEs": [
         [
-          "http://i.imgur.com/syJxavV.jpg",
+          "http://i.imgur.com/Et3iD74.jpg",
           "Una imagen que muestra nuestro certificado de desarrollo Front End",
           "Este desafío te otorga tu certificado autenticado de desarrollo Front End. Antes de que podamos emitir tu certificado, debemos verificar que has completado todos los Bonfires básicos e intermedios, y todos los Ziplines básicos e intermedios. También debes aceptar nuestro Juramento de honestidad académica. Pulsa el botón siguiente para iniciar este proceso.",
           ""