facebook og:image not working

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Facebook-API

when i try to post a comment (share link) on an article from my page which is actually a page i am using as an application in facebook : https://apps.facebook.com/ids_newsletter/ , facebook chooses the feed thumbnail image of the post feed randomly from my page. i tried using:

<meta property="og:title" content="IDS June Newsletter" />
<meta property="og:description" content="Check this out" />
<meta property="og:image" content="MYSITEURL/Images/logobig.png" />
<meta property="og:image:type" content="image/png" />
<meta property="og:image:width" content="200" />
<meta property="og:image:height" content="200" />

to put a custom image for the post but nothing affected the post thumbnail! .so what should i do ?? note that if i test the page in the fb object debugger my thumbnail appears. You can test the application and see what happens if that would help . any answers would be appreciated.i am stuck with this.