emails: Fix the image source for the follow-up day 2 email.

Copied the image from the `/help` directory to the `/emails` directory
 to use it with the `email_images_base_url` variable.

templates/zerver/emails/followup_day2.html

@@ -13,13 +13,13 @@
     {{ _("In Zulip, <b>streams</b> determine who gets a message. <b>Topics</b> tell you what the message is about.")}} {{ _("Using topics, you can read Zulip one conversation at a time. You'll see each message in context, no matter how many different discussions are going on.") }}
 </p>
 
-<img src="/static/images/help/streams-and-topics.png" alt="{{ _('Streams and topics in the Zulip app') }}"/>
+<img class="responsive-width" src="{{ email_images_base_url }}/streams-and-topics.png" alt="{{ _('Streams and topics in the Zulip app') }}"/>
 
 <p>
     {{ _("To kick off a new conversation, just pick a stream and start a new topic. This way, the new conversation thread won't interrupt ongoing discussions. For a good topic name, think about finishing the sentence: “Hey, can we chat about…?”") }}
 </p>
 
-<img src="{{ email_images_base_url }}/day2_2.png" alt="{{ _('Examples of short topics') }}"/>
+<img class="responsive-width" src="{{ email_images_base_url }}/day2_2.png" alt="{{ _('Examples of short topics') }}"/>
 
 <p>
     {% trans %}Don't stress about picking the perfect name for your topic. If anything is out of place, it's easy to <a href="{{ move_messages_link }}">move messages</a>, <a href="{{ rename_topics_link }}">rename topics</a>, or even <a href="{{ move_topic_to_different_stream_link }}">move a topic to a different stream</a>.{% endtrans %}