I used to address www.everystockphoto.com for embedding free images into my blog posts or get photos for my presentations, but I have noticed several drawbacks. In particular, embedded photos do not work with automatic crossposting to Google+. Also I tend to find fewer good images there recently.
This time, after a casual search at that website, I stumbled upon morgueFile, a free photo archive with decent images and attractive legal terms. You are free to adapt the work, to use this work for commercial purposes and without attributing the original author. The limitations are that (obviously) you cannot claim ownership of the image in its original state and that you cannot use the image exactly as it is without an alteration.
The 'stand alone basis' limitation is easy to comply with if you use Google+ as it has a neat lightweight photo editor for all images uploaded to the profile.