They say: "Finding free quality images is a tedious task - mainly due to copyright issues, attribution requirements, or simply lack of quality. This inspired us to create Pixabay - a repository for outstanding public domain images. You can freely use any image from this website in digital and printed format, for personal and commercial use, without attribution requirement to the original author. Currently there are 71888 images available: 48115 Photos + 23773 Clipart"
It's a very nice site, and to be honest, rather puts the Clip Art Lord site mentioned below to shame. I did a search on 'libraries' and got some really nice images - both photographs and icons. There are various categories that you can search in, and you can also search via photographers or cameras. You can join the site and leave comments as well. This is a site that's really worth taking some time over, and I'm pretty sure that you'll come back to it in the future when you need images for a presentation or a report.