Ad planners, researchers and other insight miners couldn't be more pleased about the democratization of digital photography and the astronomical number of pictures of the mundane that can be found online.
Looking at the pictures of people's experiences related to the brand I'm researching is a lot of fun. The way a picture is composed, the way the subject matter is centered can say a lot about what people think is important. Sometimes, we would recruit a bunch of people, hand them cameras for a week or two, and then follow up with questions. Often, though, what we do is armchair photoethnography -- going through photos already posted somewhere and looking for patterns.
My first stop used to be Flickr, but now I often start with Ginipic, a free desktop app that searches multiple photosharing sites at once. It's probably obvious in retrospect, but it took me some time to realize that the kinds of pictures people share on Flickr and, say, Picasa can often differ dramatically. Flickr, after all, is a forum for amateur and professional photographers who try to put their best foot forward, while Picasa is a place where you put your kids' snapshots for the grandma to see. Consider these search results, via Ginipic, for "Niagara": the first set is from Flickr, the second set is from Picasa. Pictures in the former are fancier, pictures in the latter have a lot more faces.