When we talk about Outlook Email Background Images - it's extremely painful. I used various email editors but background image support for outlook is not there. Here's a working solution that solves that pain:

Tools I used:
https://backgrounds.cm/

<table border="0" cellpadding="0" cellspacing="0" width="100%">
    <tr>
        <td align="center" width="100%">
            <table cellpadding="0" cellspacing="0" border="0" height="500" width="100%">
                <tr>
                    <td background="http://www.planwallpaper.com/static/images/6909249-black-hd-background.jpg" style=" background-size: cover; background-repat: no-repeat; max-height: 500px;" bgcolor="#3b4a69" valign="top">
                        <!--[if gte mso 9]>
                        <v:rect xmlns:v="urn:schemas-microsoft-com:vml" fill="true" stroke="false" style="mso-width-percent:1000;height:500px;">
                        <v:fill type="frame" src="http://www.planwallpaper.com/static/images/6909249-black-hd-background.jpg" color="#3b4a69" />
                        <v:textbox inset="0,0,0,0">
                        <![endif]-->
                        <div>
                            <!-- Overlay image or text -->
                            <table align="center" border="0" cellpadding="0" cellspacing="0" width="100%" style="max-width:800px;">
                                <tr>
                                    <td align="center" valign="top" style="padding: 60px 0 40px 0;">
                                        <img src="http://www.freeiconspng.com/uploads/success-icon-10.png" border="0" style="width: 300px; display: block;">
                                    </td>
                                </tr>
                            </table>
                        </div>
                        <!--[if gte mso 9]>
                        </v:textbox>
                        </v:rect>
                        <![endif]-->

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

I hope this helps.