Yes is true, free! And legal to use, of course you should read the terms, but you can pretty much use them anywhere.
For the classy Hi-res and more sophisticated images, I use PicJumbo. This is an awesome website with no fuzz just download. He also has a subscription service, so you can get the latest images in your inbox. Check it out here;
For more variety, from photos to vectors, I use Pixabay. They have a huge selection and the only little fuzz is to enter a captcha before downloading. Check it out here;
Enjoy, Happy Hunting