These databases offer very high-resolution images, as well as descriptive information. Some include tools that allow users to zoom, pan and rotate images before printing or saving.
In addition to a large number of architectural photographs and drawings, Artstor contains the Avery Library/GSAPP (Columbia University) Architectural Plans and Sections Collection.
Search Subjects -- Bridgeman Standard Classification -- Architecture to find thousands of high-resolution images of, chiefly, Western architecture in Bridgeman Education. There are nearly 2000 images of church architecture and more than 4000 on "ceilings, friezes, murals and plans."
The UH Digital Library features material from the Kenneth Franzheim II Rare Books Room in the Architecture and Art Library and from Special Collections in the M.D. Anderson Library. Architectural collections include Historic Houston Photographs, as well as the architectural drawings & photographs of
Donald Barthelme Kenneth Franzheim Lucian T. Hood & Burdette Keeland
When all else fails, use Google Image Search, Bing Image Search, or another image-only search engine.