I know what you might be thinking okay free stock photos so maybe there is a watermark or low resolution My answer to that is no and no. You'll get high resolution photos without a watermark.
Go to publicstock.net and start picking the photos and illustrations that best suit your site. They have a wide selection from animals to traveling and it isn't just photography and illustrations but digitally made art and even backgrounds available for your own use. publicstock.net is really a great find so head on over and check it out for yourself!