Yes, but check the individual copyright permissions before you use them.
Images that appear to be freely available on the Internet may not be free to use unless they have a Creative Commons licence granting copyright permission.
You should check the Creative Commons licence for individual images as the creators/licensors can select from a variety of permissions which can affect how you can use the image. When using any image with a Creative Commons licence you need to ensure that you credit the creator.
You can search for Creative Commons images on Flickr and Google Images.
