I find photos from all over the web. Below are some of my favorite resources:

http://unsplash.com/

• Free, open license. Mostly landscapes but a few with people here and there.

http://join.deathtothestockphoto.com/

• This is a membership site, they send you a zip file with 10 free, open license photos every month.

http://nos.twnsnd.co/

• These are vintage photos from public archives, in the public domain.

http://picjumbo.com/

• These are free and open. This site has categories you can use to narrow down to people, places, technology, etc.

http://thepatternlibrary.com

• This is a great place to find cool patterns and other visual elements (not photos).

http://www.gratisography.com/

• Free, open license. Mostly landscapes and some “artsy” stuff.

http://www.freeimages.com/

• Free, licenses on each image and usually restricted to non-commercial use.