Photos: Behind the scenes at Shutterfly's secret workshop

The end-of-year shopping season is the busiest for photo-sharing and photo-finishing services such as Shutterfly. More than half of its sales are made at the end of the year.

Unlike most e-commerce companies that lack brick-and-mortar operations, or rivals that outsource their manufacturing, Shutterfly manages its own plants to print, assemble, and ship personalized photo greeting cards, calendars, and books. Its 77,000-square-foot photo-finishing facility in Hayward, Calif., operates around the clock in the weeks leading up to the New Year.

Shutterfly is the most popular photo-printing site, enjoying a slight edge over Kodak Gallery and Hewlett-Packard's Snapfish, according to InfoTrends. ComScore ranks Shutterfly the ninth most popular photo-sharing Web site in the United States, including services that lack on-demand printing.

Custom-printed presents are the fastest-growing part of this business, as the trend of mass customization expands. Personalized gift-giving, enabled by the Web, allows people to put their unique stamp on otherwise run-of-the-mill items such as calendars, mugs, and Christmas tree ornaments.

Caption text by CNET.com Reporter Elsa Wenzel.