Most of the photos I include with my posts are found on Flickr and have Creative Commons license. Sometimes I use my own photos but the bulk of them are from Flickr.
Adding a Flickr photo to your post is pretty easy. It is just important that you select photos that have a license allowing you to use them and that you give the owner of the photo proper credit. And for me, the hardest part, is actually locating the exact photo that I want to use.