Bob is correct about Canadian vessels using Londonderry as a 'turnaround' base.
See the book
50 North An Atlantic Battleground (1963) London by Lt. Cmdr. Alan Easton RCN (ret) who was just such a person to command a Canadian escort group which used Londonderry.
Other than the IWM as Bob mentions, I would also suggest you contact the following:
Derry Harbour Museum
Harbour Square, Derry, Co Londonderry BT48 6AF, Northern Ireland
Tel: +44 (0)1504 377331, 365151
Fax: +44 (0)1504 377633
Opening days: Mon - Fri
E-mail:
museums@derrycity.gov.uk"Ship models, half-hulls, maps and pictures related to maritime heritage of Derry. The "Iona Curragh" 1963 repro of ancient boat. Prow of emigrant sailing ship "Minnehaha". Museum is in 1882 HQ building of Londonderry Port & Harbour Commissioners."
http://www.derrycity.gov.uk/museums/harbour.aspThey may quite well have some ww2 photos of the harbour and naval base.
There may also be a Londonderry public library and/or archives which may have a local history section which apart from the the above harbour museum would be the other sources I would expect to have harbour photos for you in ww2.
Also these below I noticed taken apparantly at Londonderry on the net from a very quick search:
http://www.secondworldwarni.org/Images/62a_lg.jpgU-boats alongside at Londonderry
http://www.navy.gc.ca/project_pride/all_images/photo_archive_images/Large/MC-2600.jpgHMS Mayflower alongside Collingwood & Bittersweet at Londonderry, Ireland form the Ken Macpherson Naval Museum of Alberta
There is probably more about on the net as a good starting point.
Cheers
LB