Because of copyright laws, it's not always legal to post pictures on your site. However, there are a lot of amateur photographers around the net making their pictures free for anyone to use. Pics like these are called, 'in the public domain.' Often photographs are public domain, because they are old and the copyright has expired. But now many people are making their pics available for anyone to put on their own sites.

Sydney model Stephanie Naumoska, 19, was one of 32 contestants from more than 7,000 hopefuls to make the glittering final at an event promoting “healthy, proportioned, bodies.”
“Bony or beautiful?” newspaper headlines said over photographs of a gaunt Naumoska posing in a red string bikini.
Health professionals said Naumoska, who is 1.8 m (5 ft 11 in) tall and weighs just 49 kg (108 lbs), had a body mass index of just 15.1, well under the official 18 benchmark for malnutrition.