When I'm looking for images of Flags I go to the following websites:
FOTW
Flags of the World
Flags of the world with more than 44,000 pages about flags and view more than 83,000 images of flags.
City flags
City Facts
Flags of various cities.
Clan Flags
http://www.angelfire.com/realm/dschon/clans/index2.htm
Scottish Clan flags.
Flags of All Countries
Flags of all Countries
Part of a larger site, Geographic.org that offers geographical information on various countries, this site will guide users to flags and also offers a "flag identifier" page
Flags.net
World Flag Database: Welcome
Flags of the world, including national flags, ensigns, military flags and head-of-state flags.