The IMF and World Bank do have flags, but I don't think they are ever flown at headquarters. Where I have seen them displayed publicly is on cars carrying IMF/WB officials in charge of their offices in countries where this is customary.
Chiefs of resident missions usually have official cars. When the Chief of Mission uses the car to attend official functions, the World Bank flag is displayed. Displaying a flag indicates to police or guards that the diplomat is on official business and should be permitted to pass. It also permits the aide receiving the car to know who has arrived. The flag is displayed only when the Chief of Mission is actually in the car and on the way to or from an official function.
The flag I have seen is white with the logo in light blue (close or the same as the color in the United Nations Flag). The World Bank website shows the logo as a square centered in the flag. I have only seen the flags produced as limo flags approximately 12x15".
A news story on the site of the American University of Beirut, shows the flag of the World Bank. The design contains the white square and logo inside in blue and white.
IBRD (International Bank for Reconstruction and Development) is the "World Bank". The term "World Bank" generally refers to the IBRD and IDA, whereas the World Bank Group is used to refer to all 5 institutions (IBRD, IDA, IFC, MIGA, ICSID) collectively." See: http://www.worldbank.org/ibrd
When I joined the bank in 1983, we did fly the flag at HQ. However, we have multiple versions of the flag; the one shown on this page is almost correct except that the name "World Bank" is displayed below the logo. As for the more "traditional" flags, there is one for each part of the world bank group: IBRD, IFC, ICSID, MIGA. These are rarely displayed except during our annual meetings. The current world bank logo is credited to Robert McNamara.