USC Digital Library | Gwulo: Old Hong Kong
Pre-order the new Gwulo book today to get 
special pricefree shippingsigned copies, and a free sample
Details and how to order

USC Digital Library

Thanks to Annelise for introducing the University of Southern California's digital library, which includes a good collection of Hong Kong photos.

You can use their advanced search page to use different keywords, and to narrow down the search to a specific date range.