Almost as soon as I started writing this post, I realized the futility of attempting to discuss all the possible sources of high-quality digital images of architecture in just one single blog post. So I'm not going to. Today, I'll focus solely on the architecture of the ancient western world: buildings from the ancient Near East, Egypt, Greece, and Rome.
Let's go chronologically.
Joseph MacDonnell, S.J., a professor of mathematics at Fairfield University in Connecticut, has designed a well-documented website on Mesopotamian architecture. Though the images are fairly small in size, they are clear, accurately labelled, and highlight some of the key monuments of art and architecture from this time period. The Metropolitan Museum of Art's Timeline of Art History also offers a few small-scale images of Uruk, the first known city.
For ancient Egypt, you might also want to take a look at the Timeline of Art History, which boasts an extensive collection of images and essays on the architecture of this era. And don't miss this page by Professor Frank Toker of the University of Pittsburgh.
Moving on to Greece and Rome, stoa.org's Classical Athens page is a fun place to explore photographs of ancient Athenian sites. While not always expertly shot, the pictures are well-documented in sidebar captions -- a big help for people like me who get tired of vague descriptions like, "Parthenon, general view." (General view from which direction!?!?) The Stoa Consortium also hosts a blog on issues of interest to "digital classicists." The most comprehensive collection of digital images of classical sites, however, is Perseus, which users can browse by region, date, site, period, architect, or date of building. Perseus also includes a wide variety of photographs and ground plans of Roman sites. Finally, World Images Kiosk at Cal State boasts a beautiful, if somewhat disorganized, collection of Roman art images.