The major thing Bloggers struggle with while trying to publish a post online is finding a picture that is not copyright protected. A lot of Writers and Bloggers have been sued and made to pay huge sum of money for using a picture they found online without asking permission or paying for it.
Public Domain means Works of those whose exclusive intellectual property rights have expired, have been forfeited. These are free for personal or commercial use. but permission might still be needed to use such images.
Look out for Creative Common Zero (CC0) when Downloading absolutely free Pictures.
CC0 enables scientists, educators, artists and other creators and owners of copyright- or database-protected content to waive those interests in their works and thereby place them as completely as possible in the public domain, so that others may freely build upon, enhance and reuse the works for any purposes without restriction under copyright or database law.
With 400,000 images to choose from, Stock.xchng has high quality free images for Bloggers.
Please pay close attention to picture that you need to pay for from other sites.
With over 700,000 free stock photos, all images and videos on Pixabay are released free of copyrights under Creative Commons CC0. You may download, modify, distribute, and use them royalty free for anything you like, even in commercial applications. Attribution is not required.
The only restriction is that identifiable people may not appear in a bad light or in a way that they may find offensive, unless they give their consent.