I'm facing the same issue. Added this code to my Marketo Landing page and still the image is not being pulled. This is so frustrating, I tried inserting it into the Custom Head area and into the Facebook OG area and it just doesn't work.
<meta property="og:image" content="http://www.marketo.com/_assets/uploads/what-is.PNG?20130620133033"/>
<meta property="og:image:width" content="200" />
<meta property="og:image:height" content="200" />
Do you think it's still the best answer another two years in?