We personally use the following image sites.
All of these are paid image resources meaning that you have to pay for the images, but there are several free image resources out there if you do not want to pay anything out of pocket.
Dreamstime actually has a free section.
Here are a few other free sites.
There's even a search engine for free photos here that searches across many of the other sites.
If you know of a great resource that we are missing, please let us know.
NOTE: WE do not recommend using images from Google Images unless the image licensing is very clear or that you get written permission from the website owner to use the image. Most people are fine with you using their images so long as you give credit where credit is due. :)