Fix sizing of contribute recording images on about page

Set size for large displays to 640px

templates/content-about.tpl.html

@@ -656,7 +656,7 @@ import the file you recorded earlier and edit it as you wish.
 </p>
 <p>
 <img srcset="images/contribute-audacity-record-sm.png 460w, images/contribute-audacity-record.png 640w"
-     sizes="95vw" 
+     sizes="(max-width: 460px) 95vw, 640px" 
      src="images/contribute-audacity-record.png" alt="Audacity Can Record" title="Audacity Screenshot of Recording" >
 </p>
 <blockquote>Audacity is a free, easy-to-use and multilingual audio editor and recorder for 
@@ -674,7 +674,7 @@ When you have finished editing your podcast you can export it.
 </ol>
 <p>
 <img srcset="images/contribute-audacity-export-sm.png 460w, images/contribute-audacity-export.png 640w"
-     sizes="95vw" 
+     sizes="(max-width: 460px) 95vw, 640px" 
 src="images/contribute-audacity-export.png" alt="Audacity Export Settings" title="Audacity Screenshot of an Export" >
 </p>
 
@@ -701,7 +701,7 @@ has produced a number of short shows on how to configure the audio quality.
 </p>
 <p>
 <img srcset="images/contribute-mumble-record-sm.png 460w, images/contribute-mumble-record.png 640w"
-     sizes="95vw" 
+     sizes="(max-width: 460px) 95vw, 640px" 
      src="images/contribute-mumble-record.png" alt="Audacity Can Record" title="Audacity Screenshot of Recording" >
 </p>