app/views/layouts/_mailer_header.html.erb
<table cellpadding="0" cellspacing="0" border="0" style="background: #fff; margin: 0 auto; max-width: 700px; width:100%;">
<tr>
<td>
<table cellpadding="0" cellspacing="0" border="0" style="width: 100%;">
<tr>
<td style="border-bottom: 1px solid #dadfe1; padding: 20px 0;">
<%= image_tag(image_path_for("logo_email.png"),
style: "border: 0;display: block;width: 100%;max-width: 400px",
alt: setting["org_name"]) %>
</td>
</tr>
</table>
</td>
</tr>
</table>