Add responsive mobile screenshot

app/templates/user-guide.hbs

@@ -4,14 +4,18 @@
     </div>
   <h2 class="mt-4">How to Use This Site</h2>
   <p class="mt-4">Here are a few visual aids to help you learn the basics of using this site.</p>
-  <h3>Screenshots</h3>
-  <p>(Click the image to open large in a new tab.)</p>
-  <a href="/assets/images/btr-help-image.jpg" target="_blank"><img src="/assets/images/btr-help-image.jpg" class="help-img img-fluid"></a>
 
-  <h3 class="mt-5 mb-3">Video tutorial</h3>
+  <h3 class="mt-4 mb-3">Video tutorial</h3>
     <video controls="controls" src="//btr.mrgreekgeek.com/btr_getting_started-new.mp4" class="help-video" poster="/assets/images/vid-thumb.jpg">
     Your browser does not support the HTML5 Video element.
     </video>
 
+  <h3 class="mt-4 mb-3">Screenshots</h3>
+    <p>(Click the image to open large in a new tab.)</p>
+      <picture>
+        <source media="(min-width: 991.97px)" srcset="/assets/images/btr-help-image.jpg">
+        <source media="(max-width: 991.98px)" srcset="/assets/images/BTR-mobile-help.jpg">
+        <img src="/assets/images/btr-help-image.jpg" class="help-img img-fluid">
+      </picture>
 
 </div>