When I read our country's constitution, there i have seen that the president and prime minister image is placed on head of the govt office. In that case what kind of sin for the employee who worked in this office.
No it's not shirk. Shirk is ascribing partners to Allah. Having an image of the President on the wall is not ascribing partners to Allah.
It's great to see that you are worried about being sinful and may you be rewarded by it.
No, this is not Shirk,
Here is a link If you really want to know what Shirk is.