The only way I was able to do this is via this code (TD tables). I tested in outlook client 2010. I also tested via webmail client and it worked for both.
The only things you have to do is change your_image.jpg (there are two instances of this for the same image make sure you update both for your code) and #your_color.
<td bgcolor="#your_color" background="your_image.jpg">
<!--[if gte mso 9]>
<v:image xmlns:v="urn:schemas-microsoft-com:vml" id="theImage" style='behavior: url(#default#VML); display:inline-block; position:absolute; height:300px; width:600px; top:0; left:0; border:0; z-index:1;' src="your_image.jpg"/>
<v:shape xmlns:v="urn:schemas-microsoft-com:vml" id="theText" style='behavior: url(#default#VML); display:inline-block; position:absolute; height:300px; width:600px; top:-5; left:-10; border:0; z-index:2;'>
<![endif]-->
<p>Text over background image.</p>
<!--[if gte mso 9]>
</v:shape>
<![endif]-->
</td>