Hi kec,
It's probably due to the performance-wise reason that thumbnail and preview do not show in your profile.
You can modify this behavior by going to: Performance Info and Tool > Adjust visual effect > Show thumbnail instead of icon
Moreover, please post your question to the Windows Client forum (http://social.microsoft.com/Forums/zh-HK/windowsclientzhcht/threads) instead of this Exam form.
Regards,
Colt