Thanks I tried that but somehow I am unable to set a constant image, so I tried to embed the following HTML code it worked, you can replace the image with any jpeg and png image from web.
<!-- Email Header : BEGIN -->
<table role="presentation" aria-hidden="true" aria-hidden="true" cellspacing="0" cellpadding="0" border="0" align="left" width="400" style="margin: auto;" class="email-container">
<tr>
<td style="padding: 20px 0; text-align: left">
<img src="http://i.imgur.com/jaqHrPf.png" aria-hidden="true" width="500" height="100" alt="alt_text" border="0" style="height: auto; background: #ffffff; font-family: sans-serif; font-size: 15px; line-height: 20px; color: #555555;">
</td>
</tr>
</table>
<!-- Email Header : END -->