http://www.nationalarchives.gov.uk/africa/ includes a small number of Zambian photos, apparently copyright free. Several are pre or just post independence.
How about this for a Kenneth Kaunda photo?
See also the collections for other countries - there are many from South Africa, and there is one of the "Southern Rhodesia Executive Council" in 1933.
Some aerial photos are quite interesting.The second one here is Government House, which later became state house.