How to change the photo in login button?

I think your Facebook Application isn't setup properly. In the application configuration, you should have the following settings:
Site URL: mastercook.gr/
Domain: mastercook.gr

Try that, and I think it should work better. The reason I think this is the problem is because, when I go to the application page on Facebook, the "Go To App" button goes to your Facebook Page.. not your site, like it should:
www.facebook.com/apps/application.php?id=133598320064391

Hope that helps,
Alex