You can hide an image on mobile views by following these steps:
1. Open Campaign
2. Navigate to Editor tab
3. Select Friend Share Email view
4. Switch to the HTML & CSS tab
5. Find the following piece of code:
<tr>
<td>
<img alt="{{ site_setup.name }}" width="480" src="{{ "friend_share_email_background" | localize: "tkbl_default_advocate-email-background-v2.jpg", trait: "asset" }}" class="email-background"/>
</td>
</tr>
6. Remove it or comment it out
7. Save changes