Flickr is a photo sharing website that contains thousands of photos uploaded by people from all over the world. Some of those photos are free for you to use on your blog. Those photos are protected under creative commons licenses.
Before you use photos you find on Flickr in your blog, make sure you fully understand creative commons licences. Once you completely understand the legalities of using photos taken by other people that have creative commons licenses attached to them, then you can visit the Flickr website to find photos to use on your blog.
Luckily, Flickr offers a variety of search features to help you find photos with the specific types of creative commons licenses that apply to you and your blog. You can find those photo search tools in the Flickr Creative Commons page.