I was planning on doing a list like this myself. But why reinvent the wheel? Work smarter, not harder, all that jazz.
Optimize Press put up this killer list of free image sites on their blog today. You should check it out, save it in Evernote, and never run out of images for your posts again.
Be sure to double check the copyright and attribution info as stuff changes sometimes.
By the way, if you’re not aware of it, Optimize Press is a killer WordPress theme that’s great for doing member sites and anything where you need a sales funnel. I use it on a couple of my sites. Bit of a learning curve, but it makes life a lot easier when you need to run people through a system.
Check out this site and this site of mine for some very rudimentary examples. It’s a very powerful theme. But even just the basics can give you a lot of functionality that would take a bunch of single plugins to make happen.
I totally almost forgot to put an image in this post.