Facebook - How to change the image posted for comments on my app?

I have an app with a comment box inside it, which I inserted as a social plugin

<fb:comments href="https://apps.facebook.com/myappname"></fb:comments>

When a user posts a comment using that comment box, Facebook does not show the correct 开发者_C百科image. With Facebook debug I can check that the Open Graph tags for the page of my application use the application icon as image - which is what I had expected.

Still, when a user uses this comment box, on his profile appears a random image from my app. Is there a way to fix this?