Technically, you can use any image you find using the image search engines above. However, licensing and usage terms are available with the image itself. For general search sites, such as Yahoo and Bing, you'll need to select the image and view the source site to see if it's a free use or premium image.
For other sites, the licensing terms should be displayed on the site itself. Do not just copy premium images for use on your own sites, products, or services, or you could face hefty fines if it's discovered.