picstopin.com allows you to search, manage and add free stock photos to blogs, forums, websites and other online media. We host endless free Creative Commons images from many online sources and the entire system is also available as a WordPress plugin for seamless use within the WordPress platform.
Searching for images and inserting them into blog posts or articles is designed to be very fast and convenient. Just search for images by keywords and grab the embed code to insert them into your blog. The embed code contains all necessary CC attribution, that are mandatory to include, so you don't need to contribute authors manually.
If you want, you can customize your embed code: resize the image as well as select the position in which they'd like to appear on their blog. It's then simply a case of copying the short code and pasting it into the final site.