Update index.html

Author: ezipkin

index.html

@@ -1144,7 +1144,7 @@ <h3 id="outreachPenguinChallenge">Penguin Challenge</h3>
     <h1>Penguin Challenge: An activity for K-6 STEM outreach</h1>
     <img src="assets/images/penguin_challenge2.png" width="200">
 	<img src="assets/images/penguin_challenge1.png" width="200">
-    <img src="assets/images/penguin_challenge3.png" width="200">
+    <img src="assets/images/penguin_challenge3.jpg" width="200">
 
   </section>
   <section class="content">