Issues with Flatiron template

Hi,  decided to use the Marketo Flatiron template.  But having issues with the hero image.  I created an image 640x276...when I add the image in and send test email. The image will either disappear and just have a grey back ground or 

Or it squashes the image at the top, leaving a grey space at the bottom.  Have tried putting the image link in dreamweave, which works, but if you look at it on mobile it doesn't render very well...  Any ideas please....??????