When I need images, I first go to the free public domain image sites listed below.
This site requires registration, but the images are free.
Images on the ImageAfter site are free for use in your personal or commercial work.
This site has lots of good professional quality images. Some images have limitations on usage. Be careful not to click into the DreamsTime area. DreamsTime images are NOT free to use.
Pixabay Free Images
These free royalty free and public domain images can be used without attribution in digital and printed form for personal or commercial applications. However, the Shutterstock images offered require a fee to use.
Public Domain Clipart
I like this site for illustration, because there is no registration required.
Public Domain Images
This site is not as user friendly as some others, but it does have some good images.
Public Domain Photo Sites
This site has a broad range of images. I can usually find what I need here.
Public Domain Photos
This site has more than 5,000 photos.
The are many thousands of images here. Be sure to read the Terms of Service. You may need to contact the photographer for permission.
U.S. Government Photos
Photos taken by government employees for their jobs are usually public domain works. Check the rights for the image you want to use.
Wikimedia Commons has tons of quality public domain photos and images. However, just because photos are free for use to the public does not mean there are not restrictions. The license will detail the specific rights.
Occasionally, I cannot find what I need at one of these sites. Especially when I am creating artwork for an e-book or Kindle book. At these times, I like to use Fotolia.com
Rainbow Image credit: brigwer