I've read alot about there not being a background image that doesn't have a gap in Outlook desktop versions. A fellow developer, Kurt Hartzke, developed this piece of code that has worked for us. I just wanted to share.

<!-- BPP: Big Pretty Picture -->
<table bgcolor="#000000" width="100%" height="100%" style="border-spacing:0;Margin:0 auto;">
    <tr>
        <td width="100%" background="#" valign="top" style="padding:0;display:inline-block;background-repeat:no-repeat;background-position:top center;background-size: 100% auto, auto;">

<!--[if gte mso 9]>
<v:rect xmlns:v="urn:schemas-microsoft-com:vml" fill="true" stroke="false" style="width:600px;height:180px;">
<v:fill type="frame" src="#" color="#000000"/>
<v:textbox inset="0,0,0,0">
<![endif]-->
<!-- Set width and height to actual size of image. -->

    <div>
        <div style="font-size:0;">
            <table width="100%" bgcolor="#000000" style="border-spacing:0;opacity:0.7;" class="100">
                <tr>
                    <td height="25" style="padding:0;font-size:0;line-height:25px;">&nbsp;</td>
                </tr>
<!-- place content that goes over background image here -->
                <tr>
                    <td height="25" style="padding:0;font-size:0;line-height:25px;">&nbsp;</td>
                </tr>
            </table>
        </div>
    </div>

<!--[if gte mso 9]>
</v:textbox>
</v:rect>
<![endif]--> 

        </td>
    </tr>
</table>