My use of images has changed a lot over the past year. I have now reached a point where stock image sites come in as a last resort. And when I do, clients often push back and ask me to look for something different than that cheesy, cliche, polished stock image.
So, most of my image search starts with Google Images, with the option "labelled for re-use". or I browse through some of these excellent sources of free presentation images. (Although I must say that the latter now become so popular that you start recognising images instantly because of over-use).