Some of these images are "free" -- for others there is a fee. "Royalty-free" means that you don't have to pay royalty fees for use, but you may still have to pay a fee for the image. Visit these sites with caution as the pop-up ads can be invasive and read the fine print before you use any image.
For a recent overview of over 100 stock photo and license free image sites, with an emphasis on Creative Commons and Public Domain images, see this article by Jason Acidre: 100+ Free Presentation Tools: Stock Images, Fonts, Cliparts, & Infographic Sites.
Looking for non-verbal visual qualities in your images?
Why not search by color or shape or texture?
This page offers a variety of resources related to digital images. You'll find links to search engines that specifically search for images on the web. There another list that allows you to search by visual characteristic. The box devoted to Stock Images offers a list of companies that provide images for use; sometimes the images are free and sometimes there is a cost. Other boxes introduce you to some of the image resources the library offers and some good image databases available on the web. Use the grey tabs on the left to go to pages that provide subject-related image resources.