Stock images sites are full of images that are not real (post from 2008). Images from Flickr with a creative commons license are a great alternative. But recently Instagram is taking over as a great source of images. The site searchinstagram.com allows you to search images on a desktop (Instagram is a mobile-only application). If you want to use an image in your presentation, you need to send the person who took it a message, because she owns all the rights to material posted on Intagram.
Instagram is the new Flickr.