The premier online source of high-quality Apollo images is Kipp Teague's Apollo Archive.
The Apollo Image Atlas, produced by the Lunar and Planetary Institute, includes thumbnail sheets and low-resolution scans of all the 70-mm mission photography, metric camera images, panoramic camera images, and Apollo Lunar Surface Close-up Camera images.
Michael Light has produced a beautiful photo essay called Full Moon (Alfred Knopf, New York, 1999). This extraordinary coffee-table book contains 129 glorious images from the Apollo program digitized by the author from first generation negative masters and then exquisitely processed and printed. This collection tells the story of a composite Apollo mission from the moment of engine ignition at the Cape thru to splashdown. A traveling photo exhibit containing many of the images from the book is also highly recommended. See, also, a discussion of the Apollo photographs and of the color of the Moon provided by Mike Light, specifically for the ALSJ.
As of mid-1995, NASA Johnson has released digital versions of Apollo photographs in the collection of the Public Affairs Office (PAO). These are available in the JSC Digital Images collection. All of the remaining Hasselblad images have been digitized and are available here in the various Image Libraries.
The NASA Image eXchange (NIX) provides a single point of entry to various photographic databases of six NASA centers (Langley, Glenn, Ames, Dryden, Goddard, and Johnson). NIX is currently expanding to include photos and media from additional NASA centers. Search capabilities of NIX include simple searches, complex searches, and browse searches (prebuilt searches on a number of preselected topics). NIX also provides tips to users on searching, copyright information, and a comments section. Following the creation of a prototype and beta testing, NASA Headquarters approved the release of NIX to the public as of May 15, 1997.
Other photo sources can be located via the NASA Office of Space Flight webpage.