40 Million free images!
I wrote about the public domain on my blog so I'm just going to link to that page to make things easier for me instead of rewriting everything all down again.
http://woodollar.com/royalty-free-public-domain-images
But... to cut a long story short check this out, https://pixabay.com. I'm quite clued up on the public domain, the pics on Pixabay have been dedicated to the Public by the people who made them. The pictures, vectors and video can be used for posts or whatever you may want to do with them, they have no copyright.
Pixabay is just one resource, other sites like Flickr have a section of copyright free pictures.
