Photo Gallery Forums

hello, I tried to share my web page on facebook and It do not show the picture, I used facebook debugger and there I got this message :
The 'og:image' property should be explicitly provided, even if a value can be inferred from other tags.
please tell me where should i indicate that ? i try to indicate on index.gallery.phtml " <meta property="og:image" content="http://....jpg" /> but nothing has changed...
I want to know from which file facebook read meta tags so i may indicate a picture ...

i solved , the problem was from <?php if function ...