Outlook 2007 – 2013 ignores margin and padding CSS styling when placed within an image.
You can use hspace and/or vspace instead:
<img src="http://www.htmlemailcheck.com/assets/img/og-logo.png" width="500" height="300" border="0" style="display:block; border:0;" vspace="10" hspace="10" />
Alternatively, you can add the additional spacing to the image itself.