There are hundreds, if not thousands of photos of stupas on the web. Most are traveller/tourist personal snaps on private home pages. The next largest catagory is travel companies. There seem to be a very large number specialising in Nepal and Tibet. Finally, there are the various Buddhist centres. Below are samples, partially arranged according to country.
This page needs serious upgrading. There are also photos associated with all the other links on the main pages (see index at the bottom of this page)
First, Here is a link to a gallery page at an american lama's site: scroll down to "Mandalas, stupas, sacred objects" There are photos of Mahabodhi Stupa (Bodhgaya, India), Bodhnath Stupa and Swayambhunath (Nepal). Also a nice photo of old stupas in Tibet.
Bodgaya Where Buddha acheived enlightenment. These page also contains a lot of information. (BuddhaNet)
A Walk Through Mohenjodaro Buddhist Stupa (circa 200 A.D.) From Harappa.com, glimpses of South Asia before 1947. Many photos.
The Great Stupa at Sanchi: Buddhanet has the best info, with photos
Photo of the top half of the stupa at Bodhanath
Photo essay, this one's Bodnath
This is a small stupa in Dingboche Nice photo