How to make a template email with custom background?

I'm trying to make my own HTML template with custom background to be visible in email clients on both sides of the main email content. I do it with 2 tables: the outer table defined to be 100% wide and has picture in background and the inner table is 600px wide, so I should be able to see the background around the main content, for instance in Gmail. This however doesn't happen and the outer table is sized just around the inner table so there is no background picture visible. I'm basically trying to use the same approach that I can see in the standard Zoho Campaigns email templates. Below my HTML, anyone can help?

<table id="wrapper" style="background-image:url( http://campaign-image.com/zohocampaigns/1_zc_v1_28642000005726110.png)" background=" http://campaign-image.com/zohocampaigns/1_zc_v1_28642000005726110.png" bgcolor="#00FF00" border="0" cellpadding="0" cellspacing="0" width="100%">
   <tbody>
      <tr>
         <td align="center">
            <table id="Table1" border="0" cellpadding="0" cellspacing="0" width="600">
               <tbody>
                  <tr>
                     <td align="center">
                        <p>Content</p>
                     </td>
                  </tr>
               </tbody>
            </table>
         </td>
      </tr>
   </tbody>
</table>

PS

It looks like Zoho Campaigns is wrapping my HTML into own table of 600px wide and that is why I can't see any backgrounds aside the main content - there is simply no space around.

Anyone can advise how to disable this? I.e. how do I make sure the HTML email will be sent exactly in the form that I make without additional wrapping from Zoho?

Evgeny Drokov