Beautiful images of Islamic buildings are all over the Web. Islamic Architecture, for instance, offers medium- and large-sized pictures of mosques, palaces, bathhouses, and other architectural treasures from all over the Islamic world. The website is a bit of a pain to navigate -- lots of irritating sidebar ads - but the content is well-researched and very informative.
While the pickings at Greatbuildings.com are a bit slimmer, it's still a good source for photographs of some of the most well-known examples of Islamic architecture, such as the Taj Mahal and the Great Mosque of Damascus. The images are not all well-captioned, unfortunately, so you do have to know what you're looking for.
The cream of the crop, however, is ArchNet's digital library, which offers photographs of hundreds of Islamic monuments around the world. You can search by country, time period, building usage, building style, or site name, and no matter how obscure the site, ArchNet's images are nearly always superb. Just for fun, here's on of my favorite's: the Djingareyber Mosque Restoration in Timbuktu, Mali.