Setting email images max-width in Outlook

We have created an email template that has a max width of 640, using tables and the following code to try and display the email correctly in outlook (found after research online):

 <!--[if gte mso 9]>
<v:rect xmlns:v="urn:schemas-microsoft-com:vml" fill="true" stroke="false" style="width:640;">
<v:fill type="tile" src="images/header-bg.jpg" color="#ffffff" />
<v:textbox style="mso-fit-shape-to-text:true" inset="0,0,0,0">
<![endif]-->

(email content)

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

The problem is when a user uploads an image in full size (say 1920px wide), Outlook will crop the image to match the dimensions above. Gmail and other GOOD email clients (*wink*) resize the image correctly, outlook doesn't.

I'm just wondering if anyone has any idea how I can modify this so the image isn't cropped but resizes instead? 

Thanks in advance!