Stock images are royalty-free images that are licensed for use on your website. It is important to only use images which are legally licensed so that you avoid any copyright infringement. Pricing varies widely among different paid stock image companies. A couple of my favorites are Dreamstime and iStockPhoto.
There are also a few good free stock image websites where you can find images that are licensed for commercial use. Pixabay is my favorite free stock image website with over 10,000 free images. Pexals is also worth checking out.