Facebook share link - issues defining thumbnail image on iframe app

I've created a simple Iframe app and attached it as a tab on my fanpage.

On this page I have a classic share link:

http://www.facebook.com/sharer.php?u=http://www.fb-iframe-app-url.com">SHARE IT NOW!</a>

The iframe app is hosted on my server and here ive defined what usually works for images:

<meta property="og:image" content="http://domain.com/image.gif"开发者_高级运维 />

However it seems all this kind of data, is fetched from the fanpage itself - which means I cant control those elements.

Do anyone have experience with customizing share thumbnail and text from within an iframe app-clicked share url?