The digital gallery of First World War ‘Official Photographs’ has been created by the National Library of Scotland (NLS), Edinburgh. This collection with some 4,000 photographs is originally part of the Douglas Haig papers (NLS, Acc. 3155 and MSS. 28001-). The digital collection has been divided conveniently in a number of sections, for example images of Field Marshall Earl Douglas Haig himself, the British Western Front, royal visits, images of the British cavalry, of the allied forces, and images created after the Great War. There are also photographs of Germans and images taken in Germany. You can browse the images, use a simple search with a free text field or the advanced search mode.
The NLS has also created a Flickr gallery with nearly 2,000 World War One Official British Photographs. The University of British Columbia has digitized 3,860 images from a similar collection in their holdings.