How can I get pictures to post on my site or blog?

I’d like to use pictures for each of my blog posts. I want to use these pictures without infringing on anyone’s copyright. Websites like Entrepreneur.com and pretty much every content website out there does this.

Where can I get pictures to use on my site without infringing on anyone’s copyright? Are there any websites I can use? If I get pictures from an individual do I have to ask that person to sign a release or anything like that?