Facebook OpenGraph Image - Define wall post image and different comment image?

I know it's possible to choose which image is used when a link is shared directly on to a wall, but is it possible to define which image is used when a link is shared as a comment?

I've defined two meta tags for images, one sqaure and one rectangle but only the rectangle image ever seems to show.

<meta property="og:image" content="path/to/image/facebook.jpg" />
<meta property="og:image" content="path/to/image/square-new.jpg" />

Wall post:

Comment image:

As you can see the rectangle image appears as the comment image, not the square image.

I've tried swapping the order of the meta tags and also enlarging the images but to no avail.

Is this possible?