As former President George W. Bush is no longer in office then there is absolutely no way he can be "Impeached".

Impeach (In the sense being discussed and as originally presented) - To charge (a public official) with improper conduct in office before a proper tribunal.

Impeachment In the U.S. Congress or a state legislature the presentation of charges against a public official by the lower house, with trial to be before the upper house.

Impeachment - Usage Note: When an irate citizen demands that a disfavored public official be impeached, the citizen clearly intends for the official to be removed from office. This popular use of impeach as a synonym of "throw out" (even if by due process) does not accord with the legal meaning of the word. As recent history has shown, when a public official is impeached, that is, formally accused of wrongdoing, this is only the start of what can be a lengthy process that may or may not lead to the official's removal from office. In strict usage, an official is impeached (accused), tried, and then convicted or acquitted. The vaguer use of impeach reflects disgruntled citizens' indifference to whether the official is forced from office by legal means or chooses to resign to avoid further disgrace.

Should George W. Bush have been impeached? No.
Should Barack Obama be impeached? No
Difference between the two from an international perspective? When GWB spoke Putin listened, when Barry speaks Putin has a good chuckle.